Glass Pattern Stimuli in V1 and V2

Glass pattern stimuli consisted of randomly positioned dot pairs in which dot separation and pair orientation were constant across all pairs on a given trial (Fig. 3-1). On each video frame (every 10 ms), a new set of dot pairs was plotted that was independent of the previous frame. Thus, these patterns had local spatial structure within frames but no coherent spatial structure or motion between frames. We used these dynamic patterns to randomize the positions of the dots in the pattern over time, and to minimize retinal adaptation at particular dot positions. Unless otherwise stated, all dot patterns were presented within a circular aperture.

We used two different protocols for presenting Glass pattern stimuli. In the first method (used in Chapter 3), stimuli were presented for 1 s, immediately preceded and followed by 500 ms periods of dynamic random dots (with the same mean luminance on each video frame). This allowed us to avoid contamination of the Glass pattern response by any response to the luminance change caused by the onset of the dots. An additional 2 s stimulus of purely random dots and a blank screen were included in each block of trials, from which we measured baseline responses. There was a 1.5 s period of uniform blank screen (mean gray or black depending on the experiment) between each stimulus. In the second method (used in Chapter 5), stimuli were presented in an extended sequence of 320 ms epochs with no screen blanks between them. Trials usually lasted 15 seconds, and stimuli were repeated 5 times in each trial. This allowed us to collect many repeats of each stimulus (typically 100) in a short period of time, and it similarly avoided any contamination with a luminance response because dot patterns with the same mean luminance on each video frame were presented throughout the duration of the stimulus. We determined the spontaneous response from 500 ms of blank screen preceding the stimulus.

Dots were usually presented at maximum contrast (i.e., white dots on a black background). The maximum luminance was 68.4 cd/m$^2$, and the minimum was near 0.0 cd/m$^2$. The mean luminance of the display was approximately 0.2 cd/m$^2$ when displaying white dots on a black background. All of these stimuli, in which the dots all have the same luminance, are called same-polarity patterns. We also used opposite-polarity patterns, in which half of the dots were maximum luminance (``white''), the other half were minimum luminance (``black''), and the background was mid-gray (34.2 cd/m$^2$). For opposite-polarity Glass patterns, each pair consisted of one white and one black dot. The white dot in each pair was chosen at random. Dots were square and their size was typically 0.04$^\circ$ (range, 0.03$^\circ$-0.12$^\circ$) along each side. Dot density was typically 200 dots/degree$^2$/second (range, 100-800). In this range, human observers readily perceive Glass patterns, and variations in dot density have no significant impact on perception (Alliston et al., 2001).

For each neuron, we presented Glass patterns at eight orientations ($\theta$) and five dot separations ($r$). The orientations were evenly spaced over 180$^\circ$. We chose the range of $r$ to include values from approximately $\lambda/4$ to $3\lambda/2$, where $\lambda$ was the preferred spatial period of the cell. In cells where we found the optimal $r$ at the top or bottom of our tested range we collected additional data in a range that included the best $r$ and values on either side. In all further Glass pattern experiments, we used the values of $r$ and $\theta$ determined to optimize the cell's response. When the data were noisy and the preferred $\theta$ was unclear, we chose $\theta$ to be aligned to the cell's optimal response to gratings.