The processing of edge information in visual areas of the cortex as evidenced by evoked potentials

Large-Scale Visuomotor Integration in the Cerebral Cortex

Efficient visuomotor behavior depends on integrated processing by the visual and motor systems of the cerebral cortex. Yet, many previous cortical neurophysiology studies have examined the visual and motor modalities in isolation, largely ignoring questions of large-scale cross-modal integration. To address this issue, we analyzed event-related local field potentials simultaneously recorded from multiple visual, motor, and executive cortical sites in monkeys performing a visuomotor pattern discrimination task. The timing and cortical location of four aspects of event-related activities were examined: stimulus-evoked activation onset, stimulus-specific processing, stimulus category-specific processing, and response-specific processing. Activations appeared earliest in striate cortex and rapidly thereafter in other visual areas. Stimulus-specific processing began early in most visual cortical areas, some at activation onset. Early onset latencies were also observed in motor, premotor, and prefrontal areas, some as early as in striate cortex, but these early-activating frontal sites did not show early stimulus-specific processing. Response-specific processing began around 150 ms poststimulus in widespread cortical areas, suggesting that perceptual decision formation and response selection arose through concurrent processes of visual, motor, and executive areas. The occurrence of stimulus-specific and stimulus category-specific differences after the onset of response-specific processing suggests that sensory and motor stages of visuomotor processing overlapped in time.

Neuronal generators of visual evoked potentials in humans: visual processing in the human cortex

PURPOSE

We wished to define the localization of cortical generators of visual (pattern) evoked potentials (VEP) and the temporal sequence of activation in the occipital region.

METHODS

In 4 candidates for epilepsy surgery, a large array of subdural electrodes was placed over occipital areas. Checkerboard pattern reversal stimuli were generated and the epileptogenic focus was localized and functionally mapped. Magnetic resonance imaging did not show any occipital lesions in any of the 4 patients.

RESULTS

The area first activated was the lingual gyrus in the mesial occipital lobe (negative potential peaks at approximately 70 ms), followed by an area superior to the calcarine fissure (negative peaks at approximately 80 ms). Later (starting at approximately 90 ms), there were positive potentials over the occipital pole and lingual gyrus, followed by potentials at the lateral occipital lobe.

CONCLUSIONS

These data support the idea that VEP are generated in the mesial and lateral occipital cortex by different circumscribed neuronal generators with different latencies of activation. The scalp-recorded N1 and P1 potential peaks most likely derive from the progressive activation of neuronal masses in different regions of the occipital lobe.

Pattern visual evoked potentials recorded from human occipital cortex with chronic subdural electrodes

Pattern evoked potentials to full- and partial-field stimulation were recorded simultaneously from scalp electrodes and from subdural electrodes located over the temporal and occipital cortex, including electrodes placed over or close to the lower lip of the calcarine fissure. High-amplitude pattern evoked potentials were recorded exclusively from electrodes localized in the vicinity of the calcarine fissure and showed a positive-negative deflection in phase with surface recordings, followed by a second negative peak phase reversed with respect to the major surface positive peak ("P100"). The findings suggest that the initial component is an expression of the afferent volley and that the second component (equivalent of the surface "P100") is most probably generated as a dipole strictly localized to the visual cortex in close proximity of the calcarine fissure (area 17 and/or area 18).

Brain-stem auditory evoked responses as indicators of early brain insult

The relationship between cranial ultrasonograms (SONOs) and brain-stem auditory evoked responses (BAERs) was evaluated in 2 independent samples of newborn infants at risk for brain injury (n = 113 and 203). Features of the BAER wave forms subjected to stepwise linear discriminant analysis formed the basis of an algorithm used to detect and follow early brain injury. Using this algorithm, information derived from BAERs reliably predicted SONO abnormalities at least 82.3% of the time in the initial study which was replicated with the second sample (77.3%). The wave I component latency (CL) and the wave III-V inter-peak latency interval (IPL) were independent of each other, and both contributed to a prediction of SONO abnormality. Possible mechanisms for these BAER results include compromise to the cochlear membrane or to the auditory nerve itself as well as prolongation of transmission in the brain-stem due to brain-stem hemorrhage, edema, or compression. Normative BAER values and non-linear regression functions for the wave I, III and V CLs, and the I-III, III-V, and I-V IPLs were calculated across age using data from 109 infants who demonstrated normal BAER patterns and had no history of SONO abnormalities. Our analyses indicate BAER techniques, where a single higher intensity is used to produce the BAER wave form, are both valid and efficient for use in the evaluation of early brain injury.

Selectivity of the evoked potential for vernier offset

Hyperacuity thresholds of a few arc seconds can be achieved psychophysically for a variety of spatial localization tasks. The present experiments show that evoked potentials can be elicited in response to the introduction of vernier offsets, but not by the introduction of other cues to hyperacuity such as bisection or relative pattern motion, although each of these cues is equally salient psychophysically. Moreover, vernier acuity measurements and the evoked potentials elicited in response to vernier offsets are strongly degraded by the introduction of flanking stimuli 2-4 min from the vernier target. This suggests that the hyperacuity VEP is a cortical correlate of a very specific type of hyperacuity, that produced by vernier offsets (colinearity failure).

Steady-state visual evoked responses in anesthetized monkeys

Steady-state stimuli are repeated continuously at a frequency that evokes averaged EEG waveforms with components at a frequency which is the same or some multiple of the stimulus rate. A variety of stimulus parameters are readily manipulated and controlled. We wished to assess the suitability of this approach for animal experimentation, and chose to evaluate steady-state visual evoked potential (VEP) responses to counterphased checkerboard stimuli in anesthetized adult male Rhesus monkeys. Large VEPs were reliably produced over the range of 3-6 Hz reversal rate; at 10 Hz, the VEP spectral power was clearly diminished in all monkeys. At each given reversal rate, monkeys differed substantially in variability and amount (up to 3-fold) of spectral power. These large inter-subject differences paralleled what we have seen in similar studies of unanesthetized humans. All monkeys developed large VEPs at the midline, but the topographic distribution of maximum VEP was not uniform; in 3 monkeys the power was much greater over one hemisphere than the other. Fatigue or habituation was not evident; in each monkey, there were usually no significant differences in VEP power over the 3 consecutive 30 sec segments of a given stimulation. We conclude that the anesthetized monkey can be a suitable model for study of steady-state phenomena, as long as the proper stimulus parameters are employed and as long as the level of variance is taken into account.

Application of pattern evoked potential techniques for evaluating infant perceptual systems

An evoked potential (EP) test was developed to aid in the diagnostic evaluation of a 16-week-old infant of initially unknown pathology and degree of perceptual deficit. Patterned stimuli were presented at two repetition rates to challenge the infant's visual and auditory information processing capabilities. The data were compared to those obtained from a "normal" infant of the same age and sex and, where possible, to normative population data. The EP data indicated the following: (1) the "suspect" infant's occipital response to visual pattern was abnormal in terms of waveform characteristics and in the relationship of its amplitude to spatial frequency of pattern; (2) in comparing brain response to 1 Hz and 6 Hz stimulation rates, the "suspect" infant's occipital EPs were abnormally attenuated for the faster presentation rate when a patterned visual display was part of the stimulus complex; (3) an auditory speech sound produced equivalent EP effects in both infants. These data indicated that the "suspect" infant's perceptual deficit was restricted to the processing of visual pattern information, possibly due to abnormal functioning of the geniculo-striate system. Subsequent tests and the diagnosis of ocular albinism corroborated the results of the abnormalities suggested by the EP procedure.

Activity of neurones in the inferotemporal cortex of the alert monkey

The activity of neurones in the inferotemporal cortex of the alert rhesus monkey was recorded while the monkey was shown visual stimuli, which included both food and non-food objects for comparison with the activity of neurones in the lateral hypothalamus and substantia innominata. In the anteroventral part of the inferotemporal cortex, neurones were found with visual responses which were sustained while the animal looked at the appropriate visual stimuli. The latency of the responses was 100 msec or more. The majority (96/142 or 68%) of these neurones responded more strongly to some stimuli than to others. These units usually had different responses when objects were shown from different views, and physical factors such as shape, size, orientation, colour and texture appeared to account for the responses of some of these units. Association of visual stimuli with a food reward (glucose solution) or an aversive taste (5% saline solution) did not affect the magnitude of the responses of the neurones to the stimuli either during the learning or after the period of learning. Nor did feeding the monkey to satiety affect the responses of the neurones to their effective stimuli.