During object recognition, both recognition performance and BOLD signals in object-specific areas (posterior fusiform gyrus and lateral occipital complex) increase with increasing presentation durations from 20 to 500 ms, reaching a plateau for > 120 ms. (Grill-Spector et al., 2000). By contrast, single unit responses during image categorization in the human medial temporal lobe, frontal and supplementary motor cortices have peak firing rates near 650 ms after stimulus onset (Kreiman et al., 2000).
To examine the timing of single neuron responses during categorization, we varied image duration from 300, 500, to 800 ms and studied 40 single unit responses (35 medial temporal, 1 frontal, and 4 supplementary motor) recorded from three medial temporal epilepsy patients. Between 15-20 pictures, selected from 6 image categories (animal, building, famous people, indoor scene, outdoor scene, and tools), were shown for each duration during a face/non-face categorization task. Of 40 neurons, 19 (15 medial temporal, 1 frontal, and 3 supplementary motor) showed a significant effect of duration on response magnitude, independent of the image category (2-way ANOVA, p < .05). 16 of these (only 12 medial temporal) were also independent of the specific image. These findings suggest that single neuron responses in the human medial temporal lobe, frontal and supplementary motor cortices are sensitive to the duration of images and may reflect decision making and other cognitive processes beyond simple recognition.