Strategies of visual perception suggested by optically imaged patterns of functional architecture in monkey visual cortex

Differences in spatial and temporal frequency interactions between central and peripheral parts of the feline area 18

The visual system demonstrates significant differences in information processing abilities between the central and peripheral parts of the visual field. Optical imaging based on intrinsic signals was used to investigate the difference in stimulus spatial and temporal frequency interactions related to receptive field eccentricity in the cat area 18. Changing either the spatial or the temporal frequency of grating stimuli had a significant impact on responses in the cortical areas corresponding to the centre of the visual field and more peripheral parts at 10 degrees eccentricity. The cortical region corresponding to the centre of the gaze was tuned to 0.4 cycles per degree (c/deg) for spatial frequency and 2 Hz for temporal frequency. In contrast, the cortical region corresponding to the periphery of the visual field was tuned to a lower spatial frequency of 0.15 c/deg and a higher temporal frequency of 4 Hz. Interestingly, when we simultaneously changed both the spatial frequency and the temporal frequency of the grating stimuli, the responses were significantly different from those estimated with an assumption of independence between the spatial and temporal frequency in the cortical region corresponding to the periphery of the visual field. However, in the cortical area corresponding to the centre of the gaze, spatial frequency showed significant independence from temporal frequency. These properties support the notion of relative specialization of visual information processing for peripheral representations in cortical areas.

Single units and sensation: a retrospect

The single-neuron doctrine is reexamined, and the search for causal links between single units and sensation reviewed. Although several decades of single-unit recording have been very successful in elucidating physiological mechanisms, linking signals from a single cell and perception has progressed at a slower rate. Nevertheless, analysing the activity of single neurons has achieved significant gains and remains the most promising level for elucidation of processing algorithms in the visual system. At the subcortical level, the conclusion that signals from just a single cell can carry enough information for some kinds of performance remains (almost) valid. Under carefully controlled conditions, just a few impulses in a few retinal ganglion cells are an adequate signal for the cortex to initiate a behavioural response. Elucidating cortical codes has been more difficult, and evidence exists suggesting the sharing of responsibility for a task among cell assemblies; how large these assemblies are, and how to test for them neurophysiologically, remains a challenge.