John W. McClurkin

Cortical feedback increases visual information transmitted by monkey parvocellular lateral geniculate nucleus neurons

We studied the effect of cooling the striate cortex on parvocellular lateral geniculate nucleus (PLGN) neurons in awake monkeys. Cooling the striate cortex produced both facilitation and inhibition of the responses of all neurons, depending on the stimulus presented. Cooling the striate cortex also altered the temporal distribution of spikes in the responses of PLGN neurons. Shannons information measure revealed that cooling the striate cortex reduced the average stimulus-related information transmitted by all PLGN neurons. The reduction in transmitted information was associated with both facilitation and inhibition of the response. Cooling the striate cortex reduced the amount of information transmitted about all of the stimulus parameters tested: pattern, luminance, spatial contrast, and sequential contrast. The effect of cooling was nearly the same for codes based on the number of spikes in the response as for codes based on their temporal distribution. The reduction in transmitted information occurred because the differences among the responses to different stimuli (signal separation) were reduced, not because the variability of the responses to individual stimuli (noise) was increased. We conclude that one function of corticogeniculate feedback is to improve the ability of PLGN neurons to discriminate among stimuli by enhancing the differences among their responses.

Orbital position and eye movement influences on visual responses in the pulvinar nuclei of the behaving macaque

SummaryWe studied the influences of eye movements on the visual responses of neurons in two retinotopically organized areas of the pulvinar of the macaque. Cells were recorded from awake, trained monkeys, and visual responses were characterized immediately before and after the animals made saccadic eye movements. A significant proportion of the cells were more responsive to stimuli around the time of eye movements than they were at other intervals. Other cells had response reduction. For some neurons, the change in excitability was associated with orbital position and not the eye movement. For other cells the change was present with eye movements of similar amplitude and direction but with different starting and ending positions. Here it appears that the eye movement is the important parameter. Other cells had effects related to both eye position and eye movements. In all cells tested, the changes in excitability were present when the experiments were conducted in the dark as well as in the light. This suggests that the mechanism of the effect is related to the eye position or eye movement and not visual-visual interactions. For about half of the neurons with modulations, the response showed facilitation for stimuli presented in the most responsive region of the receptive field but not for those at the edge of the field. For the other cells there was facilitation throughout the field. Thus, a gradient of modulation in the receptive field may vary among cells. These experiments demonstrate modulations of visual responses in the pulvinar by eye movements. Such effects may be part of the visual-behavioral improvements at the end of eye movements and/or contribute to spatial constancy.

Temporal Codes for Colors, Patterns, and Memories

We can see and understand complicated visual images without conscious effort. Other physical abilities, such as balancing, walking, and talking, are also effortless. One major difference between sensory and motor activity is that we can decompose our movements into a series of very small motions. This conscious decomposition allows our introspection to help us understand how we move. Unfortunately, we cannot decompose our visual perceptions into serial elements of seeing. Thus, introspection cannot help us understand how we see. The present work attempts to help us understand how the brain sees by decomposing vision into elements below our conscious perception: the activity of individual neurons.

Temporal Encoding of Visual Features by Cortical Neurons

Neurophysiological studies have shown that visual information is carried not only by which subset of neurons is active, but also by the temporal messages they carry. Recent theories of visual processing look at both temporal and spatial properties of cortical neurons to elucidate some of the higher order mechanisms that underlie perception. The spatio-temporal approach to neuronal encoding should lead beyond reductionist approaches, that look only at neuronal selectivity, to integrative approaches, that model visual information processing to understand visual function.