Visual integration has been successfully studied using variants of a contour integration task. In this task, the integrative mechanisms responsible for linking contour segments together are probed by employing stimuli with a continuous path of Gabor signals embedded in noise. Participants are typically asked to identify the direction an egg-shaped contour is pointing. Gabor elements are Gaussian-modulated sinusoidal luminance distributions that closely model the known spatial frequency processing properties of cells in area V1. Use of Gabor elements provides superior measurement of orientation sensitivity, and grouping of orientation cues, compared to stimuli with unknown effects on V1 neurons (e.g., arbitrarily constructed lines and dots). The embedded contours in stimuli employing Gabor elements cannot be detected by purely local filters or by the known types of orientation tuned neurons with large receptive fields. The long-range orientation correlations along the path of the contour can only be found by the integration of local orientation measurements into an emergent shape representation. Numerous studies using such tasks have explored the conditions under which human observers perceive or do not perceive contours. The findings support Field et al.'s (1993) concept of the "association field," in which neurons whose orientations are correlated in a manner that suggests the presence of a contour have facilitatory effects on each other, whereas neurons that encode elements whose orientation varies randomly have an inhibitory effect on each other. Moreover, findings from psychophysical studies are consistent with computational models derived from information theory, in which receptive and contextual fields interact to enhance the salience of phenomena that can be grouped based on statistical regularities. Validation of the concept of visual integration from psychophysical studies has come from tasks that manipulate one or more of the following 3 parameters: 1) signal noise ratio (Delta or Δ) which refers to ratio of the average spacing between adjacent background elements to the average spacing between adjacent contour elements; contours are more difficult to detect as the ratio decreases; 2) the orientation of contour elements; contours are more difficult to detect as the elements are jittered and the correlation between the angles of adjacent elements decreases; and 3) the spacing between contour elements -- for children, but not adults, contours are more difficult to detect as contour elements become further apart, even when Δ is kept constant by removing background elements as contour element spacing increases.
This variant of the task assesses visual integration by examining changes in contour integration/perception secondary to changes in the orientation of contour elements. In this version of the task, the ratio of the average distance between adjacent background elements to the average spacing of contour elements (Δ), which expresses the signal-noise ratio, is held constant at 0.90 for all stimuli (a level where symptomatic, state hospital patients with chronic schizophrenia can typically perceive the contour), and all stimuli contain exactly the same number of elements, and therefore are characterized by equal number of luminance changes. The critical manipulation is the degree of jitter of the contour elements. Participants are presented with blocks of trials, in increasing degree of jitter (and difficulty). Each cycle is presented four times. Participants were asked to judge whether the embedded contour was pointing to the right or the left.
Intact visual integration mechanisms should lead healthy controls to demonstrate a consistent performance decrement across conditions on the orientation manipulation version of the task. In contrast, altered visual integration should lead individuals with schizophrenia to show a steeper slope and a reduced jitter value at threshold (75% accuracy).
In this version of the task, participants are presented with 24 blocks of trials - 6 blocks repeated four times. Across the 6 blocks, the degree of jitter ranges from 0° to +/- 15-16 across 6 conditions (0, 7-8, 9-10, 11-12, 13-14, 15-16). Participants are administered 12 trials in each block, for a total of 288 trials and 48 trials at each jitter level. Participants are asked to judge whether the embedded contour is pointing to the right or the left. In addition, in each block, there are two types of catch stimuli that allow us to assess the attention level of the participants. Both types of catch trials use stimuli from the 0° jitter condition. In one, a continuous contour has been drawn in through the Gabor elements. The other type uses the original 0-degree jitter contour, but without any background elements. There is 1 catch trial of each type in each block of stimuli. The Eprime script contains instructions and practice trials.
These versions are currently being used in our test-retest reliability, and could change based on the results of that study.