Optical imaging of intrinsic signals in chinchilla auditory cortex

Mesoscopic landscape of cortical functions revealed by through-skull wide-field optical imaging in marmoset monkeys

The primate cerebral cortex is organized into specialized areas representing different modalities and functions along a continuous surface. The functional maps across the cortex, however, are often investigated a single modality at a time (e.g., audition or vision). To advance our understanding of the complex landscape of primate cortical functions, here we develop a polarization-gated wide-field optical imaging method for measuring cortical functions through the un-thinned intact skull in awake marmoset monkeys (Callithrix jacchus), a primate species featuring a smooth cortex. Using this method, adjacent auditory, visual, and somatosensory cortices are noninvasively parcellated in individual subjects with detailed tonotopy, retinotopy, and somatotopy. An additional pure-tone-responsive tonotopic gradient is discovered in auditory cortex and a face-patch sensitive to motion in the lower-center visual field is localized near an auditory region representing frequencies of conspecific vocalizations. This through-skull landscape-mapping approach provides new opportunities for understanding how the primate cortex is organized and coordinated to enable real-world behaviors.

Compensation of pulsation artifacts during optical imaging with and without cranial chamber

Functional Optical Imaging (OI) through the opened skull forms a group of Neuroimaging techniques characterized by a high temporal and spatial resolution on a meso-to macroscopic scale. State of the art OI experiments are generally difficult to execute, with a very timely surgical preparation preceding the experiment, that requires a skilled surgeon to mount a sealed imaging chamber onto the skull. The chamber reduces brain pulsation artifacts and swelling of the brain through movement restriction. In this work, we present preliminary results of a novel approach that does not rely on the usage of an imaging chamber with the goal to facilitate heavily the surgical animal preparation and to allow straightforward joint Electroencephalography - Optical Imaging recordings in the future. We carried out experiments to compare the movement restricting properties of the imaging chamber with the movement in a recording of an unconstrained and periodically irrigated brain. We used high-level image processing techniques to reduce brain pulsation artifacts and did a quantitative movement analysis of the recordings. Our results suggest that while recordings with imaging chamber show less sagittal movement, both with and without imaging chamber comprise the same lateral movements.

Sensory-Evoked Intrinsic Imaging Signals in the Olfactory Bulb Are Independent of Neurovascular Coupling

Functional brain-imaging techniques used in humans and animals, such as functional MRI and intrinsic optical signal (IOS) imaging, are thought to largely rely on neurovascular coupling and hemodynamic responses. Here, taking advantage of the well-described micro-architecture of the mouse olfactory bulb, we dissected the nature of odor-evoked IOSs. Using in vivo pharmacology in transgenic mouse lines reporting activity in different cell types, we show that parenchymal IOSs are largely independent of neurotransmitter release and neurovascular coupling. Furthermore, our results suggest that odor-evoked parenchymal IOSs originate from changes in light scattering of olfactory sensory neuron axons, mostly due to water movement following action potential propagation. Our study sheds light on a direct correlate of neuronal activity, which may be used for large-scale functional brain imaging.

Local versus global scales of organization in auditory cortex

Topographic organization is a hallmark of sensory cortical organization. Topography is robust at spatial scales ranging from hundreds of microns to centimeters, but can dissolve at the level of neighboring neurons or subcellular compartments within a neuron. This dichotomous spatial organization is especially pronounced in the mouse auditory cortex, where an orderly tonotopic map can arise from heterogeneous frequency tuning between local neurons. Here, we address a debate surrounding the robustness of tonotopic organization in the auditory cortex that has persisted in some form for over 40 years. Drawing from various cortical areas, cortical layers, recording methodologies, and species, we describe how auditory cortical circuitry can simultaneously support a globally systematic, yet locally heterogeneous representation of this fundamental sound property.

Stimulus-dependent changes in optical responses of the dorsal cochlear nucleus using voltage-sensitive dye

Optical imaging with voltage-sensitive dye was used to examine the spatiotemporal dynamics of stimulus-driven activity on the surface of the dorsal cochlear nucleus (DCN). Stimulation with tones at low to moderate levels produced localized regions of activation that were most commonly elongated rostrocaudally. The size of these activation areas expanded with increases in sound level, while their centers shifted from the lateral direction to the medial direction with increases in stimulus frequency. In contrast to the tonotopic patterns of activation evoked by tones, electrical stimulation of the DCN surface resulted in bands of activation that were elongated along the medial-lateral axis; response latencies increased with distance along these bands from the point of stimulation. Shifting the site of electrical stimulation from the rostral direction to the caudal direction induced corresponding shifts in the rostrocaudal location of the activation band; moving the electrode tip to subsurface depths resulted in loss of the elongated band. Transecting the DCN along the rostrocaudal axis midway between its medial and lateral extremities blocked propagation of the response to the half of the DCN distal to but not proximal to the stimulating electrode. The results suggest that the two modes of stimulation activated two distinct populations of neurons, one involving primarily tonotopically organized cells and the other crossing these tonotopic zones and likely representing the activation of parallel fibers. These results reveal a number of new features in the spatial patterns of tone-elicited activation that are not readily predicted by responses recorded electrophysiologically.

Theoretical limitations on functional imaging resolution in auditory cortex

Functional imaging can reveal detailed organizational structure in cerebral cortical areas, but neuronal response features and local neural interconnectivity can influence the resulting images, possibly limiting the inferences that can be drawn about neural function. Discerning the fundamental principles of organizational structure in the auditory cortex of multiple species has been somewhat challenging historically both with functional imaging and with electrophysiology. A possible limitation affecting any methodology using pooled neuronal measures may be the relative distribution of response selectivity throughout the population of auditory cortex neurons. One neuronal response type inherited from the cochlea, for example, exhibits a receptive field that increases in size (i.e., decreases in selectivity) at higher stimulus intensities. Even though these neurons appear to represent a minority of auditory cortex neurons, they are likely to contribute disproportionately to the activity detected in functional images, especially if intense sounds are used for stimulation. To evaluate the potential influence of neuronal subpopulations upon functional images of primary auditory cortex, a model array representing cortical neurons was probed with virtual imaging experiments under various assumptions about the local circuit organization. As expected, different neuronal subpopulations were activated preferentially under different stimulus conditions. In fact, stimulus protocols that can preferentially excite selective neurons, resulting in a relatively sparse activation map, have the potential to improve the effective resolution of functional auditory cortical images. These experimental results also make predictions about auditory cortex organization that can be tested with refined functional imaging experiments.

A map of periodicity orthogonal to frequency representation in the cat auditory cortex

Harmonic sounds, such as voiced speech sounds and many animal communication signals, are characterized by a pitch related to the periodicity of their envelopes. While frequency information is extracted by mechanical filtering of the cochlea, periodicity information is analyzed by temporal filter mechanisms in the brainstem. In the mammalian auditory midbrain envelope periodicity is represented in maps orthogonal to the representation of sound frequency. However, how periodicity is represented across the cortical surface of primary auditory cortex (AI) remains controversial. Using optical recording of intrinsic signals, we here demonstrate that a periodicity map exists in primary AI of the cat. While pure tone stimulation confirmed the well-known frequency gradient along the rostro-caudal axis of AI, stimulation with harmonic sounds revealed segregated bands of activation, indicating spatially localized preferences to specific periodicities along a dorso-ventral axis, nearly orthogonal to the tonotopic gradient. Analysis of the response locations revealed an average gradient of - 100 degrees +/- 10 degrees for the periodotopic, and -12 degrees +/- 18 degrees for the tonotopic map resulting in a mean angle difference of 88 degrees . The gradients were 0.65 +/- 0.08 mm/octave for periodotopy and 1.07 +/- 0.16 mm/octave for tonotopy indicating that more cortical territory is devoted to the representation of an octave along the tonotopic than along the periodotopic gradient. Our results suggest that the fundamental importance of pitch, as evident in human perception, is also reflected in the layout of cortical maps and that the orthogonal spatial organization of frequency and periodicity might be a more general cortical organization principle.

Measurement of cerebral blood volume in mouse brain regions using micro-computed tomography

Micro-computed tomography (micro-CT) is an X-ray imaging technique that can produce detailed 3D images of cerebral vasculature. This paper describes the development of a novel method for using micro-CT to measure cerebral blood volume (CBV) in the mouse brain. As an application of the methodology, we test the hypotheses that differences in CBV exist over anatomical brain regions and that high energy demanding primary sensory regions of the cortex have locally elevated CBV, which may reflect a vascular specialization. CBV was measured as the percentage of tissue space occupied by a radio-opaque silicon rubber that fills the vasculature. To ensure accuracy of the CBV measurements, several innovative refinements were made to standard micro-CT specimen preparation and analysis procedures. Key features of the described method are vascular perfusion under controlled pressure, registration of the micro-CT images to an MRI anatomical brain atlas and re-scaling of micro-CT intensities to CBV units with selectable exclusion of major vessels. Histological validation of the vascular perfusion showed that the average percentage of vessels filled was 93+/-3%. Comparison of thirteen brain regions in nine mice revealed significant differences in CBV between regions (p<0.0001) while cortical maps showed that primary visual and auditory areas have higher CBV than primary somatosensory areas.

Spatiotemporal patterns of cortical activity with bilateral cochlear implants in congenital deafness

Congenital deafness affects developmental processes in the auditory cortex. In this study, local field potentials (LFPs) were mapped at the cortical surface with microelectrodes in response to cochlear implant stimulation. LFPs were compared between hearing controls and congenitally deaf cats (CDCs). Pulsatile electrical stimulation initially evoked cortical activity in the rostral parts of the primary auditory field (A1). This progressed both in the approximate dorsoventral direction (along the isofrequency stripe) and in the rostrocaudal direction. The dorsal branch of the wavefront split into a caudal branch (propagating in A1) and another smaller one propagating rostrally into the AAF (anterior auditory field). After the front reached the caudal border of A1, a "reflection wave" appeared, propagating back rostrally. In total, the waves took approximately 13-15 ms to propagate along A1 and return back. In CDCs, the propagation pattern was significantly disturbed, with a more synchronous activation of distant cortical regions. The maps obtained from contralateral and ipsilateral stimulation overlapped in both groups of animals. Although controls showed differences in the latency-amplitude patterns, cortical waves evoked by contralateral and ipsilateral stimulation were more similar in CDCs. Additionally, in controls, LFPs with contralateral and ipsilateral stimulation were more similar in caudal A1 than in rostral A1. This dichotomy was lost in deaf animals. In conclusion, propagating cortical waves are specific for the contralateral ear, they are affected by auditory deprivation, and the specificity of the cortex for stimulation of the contralateral ear is reduced by deprivation.

Responses of auditory cortex to complex stimuli: functional organization revealed using intrinsic optical signals

We used optical imaging of intrinsic signals to study the large-scale organization of ferret auditory cortex in response to complex sounds. Cortical responses were collected during continuous stimulation by sequences of sounds with varying frequency, period, or interaural level differences. We used a set of stimuli that differ in spectral structure, but have the same periodicity and therefore evoke the same pitch percept (click trains, sinusoidally amplitude modulated tones, and iterated ripple noise). These stimuli failed to reveal a consistent periodotopic map across the auditory fields imaged. Rather, gradients of period sensitivity differed for the different types of periodic stimuli. Binaural interactions were studied both with single contralateral, ipsilateral, and diotic broadband noise bursts and with sequences of broadband noise bursts with varying level presented contralaterally, ipsilaterally, or in opposite phase to both ears. Contralateral responses were generally largest and ipsilateral responses were smallest when using single noise bursts, but the extent of the activated area was large and comparable in all three aural configurations. Modulating the amplitude in counter phase to the two ears generally produced weaker modulation of the optical signals than the modulation produced by the monaural stimuli. These results suggest that binaural interactions seen in cortex are most likely predominantly due to subcortical processing. Thus our optical imaging data do not support the theory that the primary or nonprimary cortical fields imaged are topographically organized to form consistent maps of systematically varying sensitivity either to stimulus pitch or to simple binaural properties of the acoustic stimuli.

Transcranial fluorescence imaging of auditory cortical plasticity regulated by acoustic environments in mice

Functional brain imaging using endogenous fluorescence of mitochondrial flavoprotein is useful for investigating mouse cortical activities via the intact skull, which is thin and sufficiently transparent in mice. We applied this method to investigate auditory cortical plasticity regulated by acoustic environments. Normal mice of the C57BL/6 strain, reared in various acoustic environments for at least 4 weeks after birth, were anaesthetized with urethane (1.7 g/kg, i.p.). Auditory cortical images of endogenous green fluorescence in blue light were recorded by a cooled CCD camera via the intact skull. Cortical responses elicited by tonal stimuli (5, 10 and 20 kHz) exhibited mirror-symmetrical tonotopic maps in the primary auditory cortex (AI) and anterior auditory field (AAF). Depression of auditory cortical responses regarding response duration was observed in sound-deprived mice compared with naïve mice reared in a normal acoustic environment. When mice were exposed to an environmental tonal stimulus at 10 kHz for more than 4 weeks after birth, the cortical responses were potentiated in a frequency-specific manner in respect to peak amplitude of the responses in AI, but not for the size of the responsive areas. Changes in AAF were less clear than those in AI. To determine the modified synapses by acoustic environments, neural responses in cortical slices were investigated with endogenous fluorescence imaging. The vertical thickness of responsive areas after supragranular electrical stimulation was significantly reduced in the slices obtained from sound-deprived mice. These results suggest that acoustic environments regulate the development of vertical intracortical circuits in the mouse auditory cortex.

Development of contralateral and ipsilateral frequency representations in ferret primary auditory cortex

Little is known about the maturation of functional maps in the primary auditory cortex (A1) after the onset of sensory experience. We used intrinsic signal imaging to examine the development of the tonotopic organization of ferret A1 with respect to contralateral and ipsilateral tone stimulation. Sound-evoked responses were recorded as early as postnatal day (P) 33, a few days after hearing onset. From P36 onwards, pure tone stimuli evoked restricted, tonotopically organized patches of activity. There was an age-dependent increase in the cortical area representing each octave, with a disproportionate expansion of cortical territory representing frequencies > 4 kHz after P60. Similar tonotopic maps were observed following stimulation of the contralateral and ipsilateral ears. During the first few weeks following hearing onset, no differences were found in the area of cortical activation or in the magnitude of the optical responses evoked by stimulation of each ear. In older animals, however, contralateral stimuli evoked stronger responses and activated a larger A1 area than ipsilateral stimuli. Our findings indicate that neither the tonotopic organization nor the representation of inputs from each ear reach maturity until approximately 1 month after hearing onset. These results have important implications for cortical signal processing in juvenile animals.

In vivo optical imaging of tone-evoked activity in the dorsal cochlear nucleus with a voltage sensitive dye

We investigated the use of optical imaging for observing the spatial patterns of neural activation in the dorsal cochlear nucleus (DCN) of hamsters during tonal stimulation. The patterns of activation were studied in the DCN, in vivo, following application of a voltage sensitive dye, Di-2-ANEPEQ, to the DCN surface. Beginning 60-90 min following dye application, tones were presented to the ipsilateral ear. Electrophysiological recordings after dye application revealed no significant toxicity of Di-2-ANEPEQ that affected the frequency-tuning properties of DCN neurons. We examined areas of activation in response to each of a series of test stimuli consisting of pure tones ranging in frequency from 2 to 20 kHz. For each stimulus condition, images were collected over a stimulus interval of 400 msec and averaged over 32 stimulus repetitions. These images revealed areas of activation with definable epicenters. The epicenters shifted from lateral to more medial locations on the DCN surface with increases in stimulus frequency. Comparison with electrophysiological data indicated a close parallel between the tonotopic gradient defined by optical imaging and that defined by the distribution of characteristic frequencies. The principal temporal and spatial features of these optical responses are described.

Coupling of the cortical hemodynamic response to cortical and thalamic neuronal activity

Accurate interpretation of functional MRI (fMRI) signals requires knowledge of the relationship between the hemodynamic response and the neuronal activity that underlies it. Here we address the question of coupling between pre- and postsynaptic neuronal activity and the hemodynamic response in rodent somatosensory (Barrel) cortex in response to single-whisker deflection. Using full-field multiwavelength optical imaging of hemoglobin oxygenation and electrophysiological recordings of spiking activity and local field potentials, we demonstrate that a point hemodynamic measure is influenced by neuronal activity across multiple cortical columns. We demonstrate that the hemodynamic response is a spatiotemporal convolution of the neuronal activation. Therefore, positive hemodynamic response in one cortical column might be explained by neuronal activity not only in that column but also in the neighboring columns. Thus, attempts at characterizing the neurovascular relationship based on point measurements of electrophysiology and hemodynamics may yield inconsistent results, depending on the spatial extent of neuronal activation. The finding that the hemodynamic signal observed at a given location is a function of electrophysiological activity over a broad spatial region helps explain a previously observed increase of local vascular response beyond the saturation of local neuronal activity. We also demonstrate that the oxy- and total-hemoglobin hemodynamic responses can be well approximated by space-time separable functions with an antagonistic center-surround spatial pattern extending over several millimeters. The surround "negative" hemodynamic activity did not correspond to observable changes in neuronal activity. The complex spatial integration of the hemodynamic response should be considered when interpreting fMRI data.

Isofrequency Band-like Zones of Activation Revealed by Optical Imaging of Intrinsic Signals in the Cat Primary Auditory Cortex

Neurons of similar frequency preference are arranged in isofrequency bands (IFBs) across the primary auditory cortex (AI) of many mammals. Across the AI of the cat, one of the most frequently studied species for auditory anatomy and function, we demonstrate IFB-like responses using optical imaging of intrinsic signals (OIS). Optically defined activations were extensively elongated along the dorsoventral axis of AI (the ratio of the major and minor axes was approximately 2:1), and systematically shifted as a function of stimulus frequency. The elongation of this IFB-like zone was more conspicuous at higher frequencies. In the ventral sector of the imaged field, the IFB-like zones of activation evoked at different pure tone frequencies tended to overlap extensively. Electrophysiological recording from loci within the optically defined zones of activation revealed matched responses to the frequencies used for optical imaging at 65% of these loci. The dorsoventral orientation of these zones of activation was also closely matched with the orientation of tangentially spreading intrinsic axon terminals, as revealed anatomically. The visualization of IFB-like architecture and tonotopic organization by OIS provides a basic framework for investigating the relationships of different spectral channels and between multiple acoustic parameters at a neuronal population level.

Optical imaging of intrinsic signals: recent developments in the methodology and its applications

Since optical imaging (OI) of intrinsic signals was first developed in the 1980s, significant advances have been made regarding our understanding of the origins of the recorded signals. The technique has been refined and the range of its applications has been broadened considerably. Here we review recent developments in methodology and data analysis as well as the latest findings on how intrinsic signals are related to metabolic cost and electrophysiological activity in the brain. We give an overview of what optical imaging has contributed to our knowledge of the functional architecture of sensory cortices, their development and plasticity. Finally, we discuss the utility of OI for functional studies of the human brain as well as in animal models of neuropathology.

Large-scale organization of ferret auditory cortex revealed using continuous acquisition of intrinsic optical signals

We have adapted a new approach for intrinsic optical imaging, in which images were acquired continuously while stimuli were delivered in a series of continually repeated sequences, to provide the first demonstration of the large-scale tonotopic organization of both primary and nonprimary areas of the ferret auditory cortex. Optical responses were collected during continuous stimulation by repeated sequences of sounds with varying frequency. The optical signal was averaged as a function of time during the sequence, to produce reflectance modulation functions (RMFs). We examined the stability and properties of the RMFs and show that their zero-crossing points provide the best temporal reference points for quantifying the relationship between the stimulus parameter values and optical responses. Sequences of different duration and direction of frequency change gave rise to comparable results, although in some cases discrepancies were observed, mostly between upward- and downward-frequency sequences. We demonstrated frequency maps, consistent with previous data, in primary auditory cortex and in the anterior auditory field, which were verified with electrophysiological recordings. In addition to these tonotopic gradients, we demonstrated at least 2 new acoustically responsive areas on the anterior and posterior ectosylvian gyri, which have not previously been described. Although responsive to pure tones, these areas exhibit less tonotopic order than the primary fields.

Studies of tonotopy based on wave N100 of the auditory evoked field are problematic

There is still dissension as to whether the auditory evoked field (AEF) reflects tonotopy in the auditory cortex. That notwithstanding, particularly the pronounced AEF wave occurring about 100 ms after stimulus onset (N100 m) is increasingly used for the investigation of issues such as cortical reorganization and representation of virtual pitch. Thus, it appears to be time for a critical revaluation of the supposed tonotopic organization of the N100 m generator. In the present magnetoencephalography study, the response to tonebursts of 500 ms duration, monaurally presented 60 dB above threshold, was recorded with a 37-channel axial gradiometer system over the hemisphere contralateral to the side of stimulation. The stimulus frequencies were 250, 500, 1000, and 2000 Hz. About 250 stimuli of each type were presented in random order in four independent sessions at intervals uniformly distributed between 2 and 2.8 s. An analysis of 19 hemispheres in 10 normal-hearing subjects showed a high intraindividual reproducibility, but also a substantial interindividual variability. In most cases, the dipole location either exhibited no significant frequency dependence at all, the dipoles for the four frequencies were not orderly aligned, or the data disagreed with the single-dipole model. In the few cases showing an arrangement of dipoles consistent with the assumption of an orderly tonotopic cortical map, the most relevant coordinate varied from subject to subject. Regarding theses results, it seems crucial to understand wave N100 m on the basis of individual subjects, whereas conclusions relying on mean dipole locations for groups of subjects are problematic.

Optical imaging of intrinsic signals induced by peripheral nerve stimulation in the in vivo rat spinal cord

We examined neural response patterns evoked by peripheral nerve stimulation in in vivo rat spinal cords using an intrinsic optical imaging technique to monitor neural activity. Adult rats were anesthetized by urethane, and laminectomy was performed between C5 and Th1 to expose the dorsal surface of the cervical spinal cord. The median, ulnar, and radial nerves were dissected, and bipolar electrodes were implanted in the forelimb. Changes in optical reflectance were recorded from the dorsal cervical spinal cord in response to simultaneous stimulation of the median and ulnar nerves using a differential video acquisition system. In the region of the cervical spinal cord, intrinsic optical signals were detected between C5 and Th1 at wavelengths of 605, 630, 730, 750, and 850 nm: the image with the largest signal intensity and highest contrast was obtained at 605 nm. The signal intensity and response area expanded with an increase in the stimulation intensity and varied with the depth of the focal plane of the macroscope. The intrinsic optical response was mostly eliminated by Cd(2+), suggesting that the detected signals were mainly mediated by postsynaptic mechanisms activated by sensory nerve fibers. Furthermore, we succeeded in imaging neural activity evoked by individual peripheral nerve stimulation. We found that the response areas related to each peripheral nerve exhibited different spatial distribution patterns and that there were animal-to-animal variations in the evoked neural responses in the spinal cord. The results obtained in this study confirmed that intrinsic optical imaging is a very useful technique for acquiring fine functional maps of the in vivo spinal cord.

Optical imaging of intrinsic signals in ferret auditory cortex: responses to narrowband sound stimuli

This paper describes optical imaging of the auditory cortex in the anesthetized ferret, particularly addressing optimization of narrowband stimuli. The types of sound stimuli used were tone-pip trains and sinusoidal frequency and amplitude modulated (SFM and SAM) tones. By employing short illumination wavelengths (546 nm), we have successfully characterized the tonotopic arrangement, in agreement with the well-established electrophysiological tonotopic maps of the ferret auditory primary field (AI). The magnitude of the optical signal increased with sound level, was maximal for a modulation frequency (MF) of 2-4 Hz, and was larger for tone-pip trains and SFM sounds than for SAM sounds. Accordingly, an optimal narrowband stimulus was defined. Thus optical imaging can be used successfully to obtain frequency maps in auditory cortex by an appropriate choice of stimulus parameters. In addition, background noise consisting of 0.1-Hz oscillations could be reduced by introduction of blood pressure enhancing drugs. The optical maps were largely independent of 1) the type of narrowband stimulus, 2) the sound level, and 3) the MF. This stability of the optical maps was not predicted from the electrophysiological literature.

Arterial blood supply to the auditory cortex of the chinchilla

Utilizing optical imaging we identified and named the arteries that supply the primary auditory cortex in the chinchilla (Chinchilla laniger). The primary auditory cortex is located 2-3 mm caudal to the medial cerebral artery and is supplied by it. Using corrosion casts and scanning electron microscopy we visualized the capillary networks in the auditory cortex and found regional variations in the densities of the capillary bed. We hypothesize that the uneven capillary densities observed in the auditory cortex correspond to neurologically more active areas.

Spontaneous and stimulus-evoked intrinsic optical signals in primary auditory cortex of the cat

Spontaneous and tone-evoked changes in light reflectance were recorded from primary auditory cortex (A1) of anesthetized cats (barbiturate induction, ketamine maintenance). Spontaneous 0.1-Hz oscillations of reflectance of 540- and 690-nm light were recorded in quiet. Stimulation with tone pips evoked localized reflectance decreases at 540 nm in 3/10 cats. The distribution of patches "activated" by tones of different frequencies reflected the known tonotopic organization of auditory cortex. Stimulus-evoked reflectance changes at 690 nm were observed in 9/10 cats but lacked stimulus-dependent topography. In two experiments, stimulus-evoked optical signals at 540 nm were compared with multiunit responses to the same stimuli recorded at multiple sites. A significant correlation (P < 0.05) between magnitude of reflectance decrease and multiunit response strength was evident in only one of five stimulus conditions in each experiment. There was no significant correlation when data were pooled across all stimulus conditions in either experiment. In one experiment, the spatial distribution of activated patches, evident in records of spontaneous activity at 540 nm, was similar to that of patches activated by tonal stimuli. These results suggest that local cerebral blood volume changes reflect the gross tonotopic organization of A1 but are not restricted to the sites of spiking neurons.

Three distinct auditory areas of cortex (AI, AII, and AAF) defined by optical imaging of intrinsic signals

Using pure-tone sound stimulation, three separate auditory areas are revealed by optical imaging of intrinsic signals in the temporal cortex of the chinchilla (Chinchilla laniger). These areas correlate with primary auditory cortex (AI) and two secondary areas, AII and the anterior auditory field (AAF). We have distinguished AI on the basis of concurrent single-unit electrophysiological recording; neurons within the AI intrinsic signal region have short (<15 ms) onset-response latencies compared with neurons recorded in AII and the AAF. Within AI, AII, and AAF we have been able to define cochleotopic or tonotopic organization from the differences in intrinsic signal areas evoked by pure tones at octave-spaced frequencies from 500 Hz to 16 kHz. The maps in AI and AII are arranged orthogonal to each other.