Average evoked potential correlates of two-flash perceptual discrimination in cats

Information persistence evaluated with low-density dot patterns

After more than a century of study, we do not yet fully understand how shapes and patterns are encoded and identified. Greater progress might result from quantifying stimulus information, thus allowing manipulation of the degree to which a shape or pattern can elicit recognition. The present work used discrete dot patterns that are seen as letters of the alphabet. By adjusting the density of the dots in each pattern, one can determine the probability that it will be recognized. The experiments displayed low-density dot patterns to human respondents, assessing the interval across which non-redundant information provided by two compatible subsets would combine to elicit recognition. This provided a measure of the time required for decay of information persistence. Viewed in the context of prior work, the evidence indicates that the retina mediates initial visibility of the stimulus trace, but the longer-duration persistence required for memory retrieval is mediated by visual cortex.

The human Retinal Functional Unit

It has long been known that readers of this page will move their eyes from one fixation to the next two to four times per second. It follows from this fact that each fixation triggers a unique optic nerve volley lasting up to 300 ms that contains all the information the retina processes between fixations. Here we give such volleys a name (Retinal Functional Unit, RFU) and use human subjects and interstimulus interval (ISI) experiments to define some of their properties. We report that RFUs can be dissected into an initial fraction that reaches the cortex and a later fraction that may not, depending on the ISI between successive stimuli. During the dissection process the perceptions of the stimuli change in an orderly way, such that successive thresholds of "twoness", color, and duration are reached as a function of increasing ISI. We conclude that volleys from the tens or hundreds of thousands of active axons contained in every RFU exit the retina in a precisely determined temporal order, and add this conclusion to three others for which we have already published the supporting data. 1) The mammalian retina normally takes about 300 ms to process a visual stimulus. 2) The ca. 300 ms end product, an RFU, contains in neuronal form all the photochemical information acquired during one fixation. 3) These information-rich volleys reach the cortex with little or no change thanks to monosynaptic transfer in the thalamus.

Two-flash threshold, sensitivity and beta in normal subjects and schizophrenics

Two-flash threshold, response criterion, and sensitivity (defined by the slope of the psychophysical ogive), were examined in schizophrenics and in normals under three conditions—a base-line condition, and during and after continuous white noise. Schizophrenics were subdivided into four groups on the basis of whether or not they exhibited paranoid symptomatology and electrodermal orienting responses. In general schizophrenics had more lenient two-flash response criteria than normal subjects. In the base-line condition schizophrenics with orienting responses had lower two-flash thresholds than those without orienting responses. Paranoid schizophrenics had more lenient criteria than non-paranoid schizophrenics and lower sensitivity than the non-paranoid group with orienting responses. Differential group effects were obtained during and after noise, especially with regard to sensitivity.