Illusory contours and cortical neuron responses Perla B. Gámez University of Chicago Sensation and Perception Autumn 2004 T.A.: Pascal Wallisch

What are Contours? Contours Object boundaries Edges Physical discontinuity in the image Ex. due to color, luminance difference between object and background

What Are Illusory Contours? Contours that are perceived even though they are not present in the physical world. Ex. In the absence of discontinuity in the stimulus.

Illusory Contour Example

Question How are Illusory Contours represented in the brain?

Alternatives In V1 Other areas involved Illusory contours phenomenon involves only area V1. Other areas involved Illusory contours phenomenon involves areas V2 or later.

Alternatives Single-cell phenomenon Not a single-cell phenomenon Single cells in area (ex. in V1; in V2) that respond to illusory contours. Not a single-cell phenomenon A population of cells in particular area (ex. in V1; in V2) collectively respond to illusory contours

Alternatives Property of Classical Receptive Fields 0.25° to 1° (ex. in V1) Property of Non-classical Receptive Fields Ex. Much Bigger Receptive Fields ≈ 2-3 X’s (ex. in V2)

Logic Rationale for methodology Start with simplest method: Using a visual fixation task, observe and measure neuronal firing. If firing occurs outside of V1, then other areas are involved, not V1. If single cell fires, then evidence for single-cell phenomenon. If firing occurs for images outside of the conventional RF, then RF are bigger.

Methods Participants Rhesus Monkeys (Macaca mulatta)

Methods Monkeys were awake, NOT anesthetized. Immobilized by implanting a bolt in the skull Single units were recorded with microelectrodes inserted through the intact dura. Trained to perform a visual fixation task. Received reward if they pulled a lever when a fixation target appeared and released it when they detected a 90° turn of the target. Target consisted of two parallel short lines

Results

Results Illusory contours “perceived” in area 18 (V2/pre-striate cortex). Cells in area 17 did not show responses related to contour, but 13 of 38 cells in area 18 did (Fig. 2, B and G). As Fig. 2 A shows, neuron 1 responded to the lower right edge of a light bar.

Results A strip covering cell’s response field was blanked out and the neuron responded to the stimulus as it did to edge of bar, only less strongly (Fig. 2 B). Neither half of the stimulus excited the neuron (Fig. 2 C & D) Parallel with perception: Illusory contours disappear when only part of the inducing configuration is viewed. Evidence for Gestalt principles: The whole is greater than the sum of its parts.

Results Small changes in configuration had effects on illusory contours. Ex. Responded well to Fig. 2 F, and 2 G, but not to Fig 2 H.

Implications Illusory contours phenomenon involves areas 18 (V2/pre-striate cortex). Single cells in area V2 responds to illusory contours: Single-cell phenomenon. Unconventional property of Receptive Fields: Bigger Receptive Fields ≈ 2-3 X’s (ex. in V2).

Problems Researchers assume that monkeys can see illusory contours. Should train monkeys to respond to contours. Ex. Press button if can see, instead of just fixating.