Delayed pattern discrimination in patients with unilateral temporal lobe damage

Abstract

Behavioral and neurophysiological studies in macaque monkeys suggest a role of the inferior temporal cortex in pattern discrimination and visual memory. To determine whether this cortical area is also involved in human short-term visual memory, we measured spatial frequency discrimination thresholds for sequentially presented stimuli in 17 patients with unilateral, postoperative focal damage to the temporal cortex (11 left, 6 right hemisphere). These results are compared to those of 17 age-matched control subjects. Contrast detection thresholds and difference thresholds for spatial frequency were determined for spatially truncated sine wave gratings presented in the left and right visual fields. Detection thresholds were measured for sine wave gratings in a spatial two-alternative forced-choice procedure for three spatial frequencies [2.5, 5, and 10 cycles (c)/degree] for each hemifield. Discrimination thresholds were determined for two gratings sequentially presented either 4° to the left or right of fixation. Grating contrast was five times the value of detection threshold and reference frequency was 5 c/degree. Within each trial, the gratings were separated in time by 1, 3, and 10 sec interstimulus intervals (ISIs), and subjects signaled which grating had the higher spatial frequency. The results indicate that (1) contrast detection thresholds overall did not differ between patient and control groups; (2) spatial frequency discrimination thresholds were, however, significantly elevated in patients and this elevation was significantly more pronounced in the visual field contralateral to the damaged hemisphere; and (3) patients with inferotemporal damage exhibited higher discrimination thresholds for the longest ISI, whereas patients with medial/superior temporal lobe damage did not show this effect. The results suggest that visual areas in human temporal cortex are involved in the higher visual processes underlying delayed pattern discrimination.