Speed of visual processing increases with eccentricity

Abstract

The visual system has a duplex design to meet conflicting environmental demands: the fovea has the resolution required to process fine spatial information, but the periphery is more sensitive to temporal properties. To investigate whether the periphery's sensitivity is partly due to the speed with which information is processed, we measured the full timecourse of visual information processing by deriving joint measures of discriminability and speed, and found that speed of information processing varies with eccentricity: processing was faster when same-size stimuli appeared at 9 degrees than 4 degrees eccentricity, and this difference was attenuated when the 9 degrees stimuli were magnified to equate cortical representation size. At the same eccentricity, larger stimuli are processed more slowly. These temporal differences are greater than expected from neurophysiological constraints.

Figure 1

SAT methodology and hypothetical functions. (a) Sequence of events in a single trial. The stimuli were randomly presented at 8 equidistant locations from a fixation point on an at either 4 or 9° eccentricity. The interval between the cue onset and the stimulus offset was brief enough to prevent goal or target directed eye movements,. Observers were required to respond within 300 ms of the tone whether the target was tilted to the right or the left. Feedback was provided after each trial and block. (b) Hypothetical SAT functions plotted in d′ units (√2 of the standard normal deviate of the probability of correctly judging the target’s orientation) versus processing time (time of the response cue plus observer’s average latency to respond). Top, expected pattern if the functions differ in asymptotic accuracy, but are associated with the same intercept and proportional rate of information accrual. Bottom, one expected pattern if the functions differ in speed of information accrual only. The intercept (d′ = 0) measures the minimal time needed for above-chance discrimination. The rate of rise of the function indexes the rate of information accrual directly,. A difference in either rate or intercept will result in disproportional SAT dynamics, so that the functions will reach a given proportion of their respective asymptotes at different times. The lines that intersect the ordinate and abscissa show the time when the functions reach the 1 −1/e (63%) point. The circles show the corresponding reaction time (RT) points in SAT coordinates, illustrating that RT differences can arise from differences in discriminability (top) or dynamics (bottom). The position of the RT points on the corresponding SAT functions are determined by the decision criteria that an observer uses to balance speed and accuracy.

Figure 2

SAT results and speed of visual processing. (a) Average discrimination accuracy (in d′) as a function of processing time. Smooth functions show the best-fitting exponential model (equation 1) for 4° (solid lines) and 9° (dashed lines) conditions. The best-fitting model allocated a separate asymptotic parameter (λ) to each of the 4 conditions, a common rate (β) parameter, and one intercept (δ) parameter to 4° and another to 9° eccentricity. Adjusted-R² was 0.95. (b) Temporal dynamics of feature searches. Accuracy normalized by asymptote (λ) isolates the differences in processing speed for feature searches at 4°, 9° and 9°-magnified (average over set sizes 1 and 8). Differences in processing speed are illustrated by plotting the time at which each condition reaches a given proportion of its asymptote, using the best fitting exponential parameters (Supplementary Methods).