Effects of Increments and Decrements of Light on Neural Discharge Rate

Evaluating the Talbot-Plateau law

The Talbot-Plateau law asserts that when the flux (light energy) of a flicker-fused stimulus equals the flux of a steady stimulus, they will appear equal in brightness. To be perceived as flicker-fused, the frequency of the flash sequence must be high enough that no flicker is perceived, i.e., it appears to be a steady stimulus. Generally, this law has been accepted as being true across all brightness levels, and across all combinations of flash duration and frequency that generate the matching flux level. Two experiments that were conducted to test the law found significant departures from its predictions, but these were small relative to the large range of flash intensities that were tested.

Luminance potentiates human visuocortical responses

Our visual system is tasked with transforming variations in light within our environment into a coherent percept, typically described using properties such as luminance and contrast. Models of vision often downplay the importance of luminance in shaping cortical responses, instead prioritizing representations that do not covary with overall luminance (i.e., contrast), and yet visuocortical response properties that may reflect luminance encoding remain poorly understood. In this study, we examined whether well-established visuocortical response properties may also reflect luminance encoding, challenging the idea that luminance information itself plays no significant role in supporting visual perception. To do so, we measured functional activity in human visual cortex when presenting stimuli varying in contrast and mean luminance, and found that luminance response functions are strongly contrast dependent between 50 and 250 cd/m², confirmed with a subsequent experiment. High-contrast stimuli produced linearly increasing responses as luminance increased logarithmically for all early visual areas, whereas low-contrast stimuli produced either flat (V1) or assorted positive linear (V2 and V3) response profiles. These results reveal that the mean luminance information of a visual signal persists within visuocortical representations, potentially reflecting an inherent imbalance of excitatory and inhibitory components that can be either contrast dependent (V1 and V2) or contrast invariant (V3). The role of luminance should be considered when the aim is to drive potent visually evoked responses and when activity is compared across studies. More broadly, overall luminance should be weighed heavily as a core feature of the visual system and should play a significant role in cortical models of vision.NEW & NOTEWORTHY This neuroimaging study investigates the influence of overall luminance on population activity in human visual cortex. We discovered that the response to a particular stimulus contrast level is reliant, in part, on the mean luminance of a signal, revealing that the mean luminance information of our environment is represented within the visual cortex. The results challenge a long-standing misconception about the role of luminance information in the processing of visual information at the cortical level.

History-dependent excitability as a single-cell substrate of transient memory for information discrimination

Neurons react differently to incoming stimuli depending upon their previous history of stimulation. This property can be considered as a single-cell substrate for transient memory, or context-dependent information processing: depending upon the current context that the neuron "sees" through the subset of the network impinging on it in the immediate past, the same synaptic event can evoke a postsynaptic spike or just a subthreshold depolarization. We propose a formal definition of History-Dependent Excitability (HDE) as a measure of the propensity to firing in any moment in time, linking the subthreshold history-dependent dynamics with spike generation. This definition allows the quantitative assessment of the intrinsic memory for different single-neuron dynamics and input statistics. We illustrate the concept of HDE by considering two general dynamical mechanisms: the passive behavior of an Integrate and Fire (IF) neuron, and the inductive behavior of a Generalized Integrate and Fire (GIF) neuron with subthreshold damped oscillations. This framework allows us to characterize the sensitivity of different model neurons to the detailed temporal structure of incoming stimuli. While a neuron with intrinsic oscillations discriminates equally well between input trains with the same or different frequency, a passive neuron discriminates better between inputs with different frequencies. This suggests that passive neurons are better suited to rate-based computation, while neurons with subthreshold oscillations are advantageous in a temporal coding scheme. We also address the influence of intrinsic properties in single-cell processing as a function of input statistics, and show that intrinsic oscillations enhance discrimination sensitivity at high input rates. Finally, we discuss how the recognition of these cell-specific discrimination properties might further our understanding of neuronal network computations and their relationships to the distribution and functional connectivity of different neuronal types.

RESPONSES OF SINGLE CELLS IN VISUAL SYSTEM TO SHIFTS IN THE WAVELENGTH OF LIGHT

Spectrally opponent cells of the macaque lateral geniculate are very sensitive to shifts from one wavelength to another, independent of the relative intensities of the different wavelengths. Shifts in opposite spectral directions from the adaptation wavelength produce opposite changes in firing rate, regardless of the particular wavelengths involved; however, any given cell is more sensitive to shifts in some spectral regions than in others.

On and off systems in human vision

Three experiments examined the interaction of On and Off responses that were produced by sudden increments and decrements in luminance. All three experiments utilized a masking technique that required observers to detect a signal in a masking field. The mask was produced by brightening or dimming a field of dots, and the signal consisted of the addition or subtraction of a dot. Experiment 1 showed that detection of the signal-dot was more difficult when the luminance of the signal and the mask changed in the same direction (e.g. a new dot added to the field of dots that were being brightened) than when luminance changed in different directions. When the amplitude of signal and mask was varied parametrically (Experiments 2 and 3), accuracy increased with the ratio of amplitudes of signal and mask. But at any given ratio, the signal was more difficult to detect when signal and mask were of the same sign. The greater difficulty encountered in detecting a signal in the context of a "like" mask is ascribed to greater interference between signal and mask when they share the channel.

The luminance and response range of monkey retinal ganglion cells to white light

The responses of single retinal ganglion cells of the monkey to spot stimuli were recorded with extracellular microelectrodes. The stimulus was centered on the receptive field, adjusted in diameter to optimally excite the cell and incremented and decremented from the background level. The response range was defined as the difference between the maximum and minimum response amplitude evoked with a wide range of luminances. The difference in luminance between the luminances which evoked the maximum and minimum responses was defined as the luminance range for dynamic response. The response amplitude range and luminance range to white light were negatively correlated to one another for the population of cells studied in central vision. The phasic cells had smaller luminance ranges for dynamic responses and a greater response magnitude for small changes in luminance from background than the tonic cells.

Effects of subcortical lesions on visual intensity discriminations in rats

The role of several subcortical structures in visual intensity discrimination was examined by comparing the effects of localized lesions on a variety of intensity discriminations. In Experiment 1 light avoidance was unimpaired after lesions of the ventral lateral geniculate nucleus (LGNv), nucleus lateralis posterior (TLP), nucleus posterior of Gurdijian (NPG), dorsal pretectum (PTd), and ventral pretectum (PTv). The LGNv, TLP, NPG and PTv, but not the PTd, groups were impaired on a simultaneous black versus white (BW) discrimination in Experiment 2. None of these groups was impaired on a horizontal versus vertical discrimination (HV). The TLP group showed a transient impairment on a successive light versus dark discrimination, not present with the LGNv and NPG groups (Experiment 3). In Experiment 4 all three groups were impaired on a successive BW discrimination. In Experiment 5 rats with LGNv lesions but not with TLP lesions had elevated relative brightness thresholds. Both groups had normal absolute thresholds. The results are related to the possibility that information about intensity and pattern is coded in separate visual pathways.

Effects of retinal adaptation on brightness estimation in humans: temporal considerations

Human subjects were asked to identify one of seven possible luminance levels under Ganzfeld conditions. The luminance range investigated was 0.5–500.0 ft 1m. Earlier findings on the subjective effects of a Ganzfeld on humans were verified. Subjects performed significantly better under non-adapted than under adapted conditions. Under both conditions subjects tended to perform better after pupillary diameter had been fixed. Errors never exceeded 1 log10 unit in magnitude.

Studies of temporal frequency adaptation in visual contrast sensitivity

1. A short adaptation to sinusoidal flicker produces a temporary elevation in the temporal contrast threshold of a human observer.2. The frequency specificity of this adaptation effect is much less than that observed with adaptation to spatial frequencies; thus it does not seem warranted to postulate the existence of distinct channels for the detection of specific temporal frequencies, as has been done in the case of spatial frequencies (Blakemore & Campbell, 1969).3. At low frequencies, a substantial adapting modulation is necessary to produce an effect, but at high frequencies an effect can be seen even with adaptation which is below threshold (as determined by the method of adjustment).4. This subthreshold adaptation appears to explain the observation that thresholds set by the method of adjustment rise by as much as a factor of two during the first minute of exposure.5. No interocular transfer of the adaptation effect was observed.6. Adaptation first appears at mesopic luminances, but its appearance is not dependent on the change from rod to cone vision. Under several conditions, however, the first appearance of flicker adaptation did coincide with a change in the deLange curve, which is attributable to the appearance of the antagonistic surround of visual receptive fields. Thus it was hypothesized that the surround is essential for adaptation.