Lack of orientation specific adaptation to vertically oriented Glass patterns in human visual cortex: an fMRI adaptation investigation

Abstract

The perception of coherent form configurations in natural scenes relies on the activity of early visual areas that respond to local orientation cues. Subsequently, high-level visual areas pool these local signals to construct a global representation of the initial visual input. However, it is still debated whether neurons in the early visual cortex respond also to global form features. Glass patterns (GPs) are visual stimuli employed to investigate local and global form processing and consist of randomly distributed dots pairs called dipoles arranged to form specific global configurations. In the current study, we used GPs and functional magnetic resonance imaging (fMRI) adaptation to reveal the visual areas that subserve the processing of oriented GPs. Specifically, we adapted participants to vertically oriented GP, then we presented test GPs having either the same or different orientations with respect to the adapting GP. We hypothesized that if local form features are processed exclusively by early visual areas and global form by higher-order visual areas, then the effect of visual adaptation should be more pronounced in higher tier visual areas as it requires global processing of the pattern. Contrary to this expectation, our results revealed that adaptation to GPs is robust in early visual areas (V1, V2, and V3), but not in higher tier visual areas (V3AB and V4v), suggesting that form cues in oriented GPs are primarily derived from local-processing mechanisms that originate in V1. Finally, adaptation to vertically oriented GPs causes a modification in the BOLD response within early visual areas, regardless of the relative orientations of the adapting and test stimuli, indicating a lack of orientation selectivity.

Figure 1

(A) Representation of the stimuli used. The Glass patterns (GPs) represented have 100% coherence (i.e. all dipoles are oriented according to the main orientation axis). From left to right: vertically oriented GP (orientation 0°). Vertically oriented GPs were used as adapting and test GPs, test GP oriented at 7.5°, test GP oriented at 30°, and test GP oriented at 90° (horizontal). The second and third GPs are tilted clockwise from vertical. (B) Schematic representation of the fMRI block design. (C) Representation of one block. See text for more details. For demonstrative purposes, the density of all the GPs represented has been reduced and the dot size increased.

Figure 2

Boxplots of relative percent BOLD signal change for the test and ROI regressors. Each test condition is represented by a distinct color. For each boxplot, the horizontal black line indicates the median and the lower and upper hinges correspond to the first and third quartiles (i.e. the 25th and 75th percentiles). The black point within each boxplot represents the mean response amplitude. The grey points represent outliers.

Figure 3

Boxplots of adaptation index values for the test orientations at 0° (parallel), 7.5°, and 30°. Please note that the reason the 90° orientation is not included in the comparison of test orientations is because it was specifically used to calculate the AI. The boxplots of each visual area are grouped together. The black point within each boxplot represents the mean adaptation index, whereas the grey points represent outliers.