Exploring the mechanisms underlying surface-based stimulus selection

Abstract

Valdes-Sosa, et al. (2000) introduced a transparent-motion design that provides evidence of surface-based processing of visual motion. We show that this design suffers from a motion-duration confound that admits an alternative explanation based on neuronal adaptation and competition. We tested this explanation by reversing the relationship between motion duration and which perceptual surface was "cued". We also examined the role of color duration. Our findings support the surface-based account and, more specifically, demonstrate that this type of surface-based selection involves selective spatial processing at the scale of the texture elements that define the transparent surfaces.

Figure 1. Delayed-onset design

A) Conventional depiction. Two superimposed dot fields rotate in opposite directions (about a central fixation target) yielding a perception of two transparent surfaces. One rotating dot field appears first followed by the “delayed” dot field. Subsequently, either the delayed (i.e. “cued”) or non-delayed (i.e. “uncued”) dot field translates briefly. After translation, both dot fields rotate. Subjects report the direction of the translation. Translations of the cued dot field are judged more accurately than translations of the uncued dot field. B) Feature-based depiction. Dot fields are distinguished by line style (dashed or solid), dot field color is given by line color, and type of motion (i.e. clockwise, counterclockwise, or translation) is given by vertical line placement. The onset differences in this design result in “cued” translations occurring in the presence of the older rotation direction and “uncued” translations occurring in the presence of the newer rotation direction (gray region).

Figure 2. Two-translation task of Valdes-Sosa et al. (2000)

A) Conventional depiction. Fixation-point color (green as in upper panels or red as in lower panels) indicates which surface translates first. Following a period of rotation, the cued dot field translates briefly, while the other field continues to rotate. The dot fields then continue to rotate for a variable delay, at which point one dot field, chosen randomly, translates briefly. After this second translation, both surfaces rotate. Observers report the direction of each translation. It was found that the first translation is judged accurately, but the second translation is only judged accurately if it is of the same dot field that translated first. B) Feature-based depiction. Conventions are same as in Fig 1B. The first translation has been proposed to exogenously cue attention to the translating dot field (Reynolds et al. 2003). This first translation also leads to a motion-duration confound (gray region): cued second translations occur in the presence of the older (i.e. non-interrupted) rotation and uncued second translations occur in the presence of the newer (i.e. interrupted) rotation.

Figure 3. Motion-competition explanation

A) Adapting responses to CW (green) and CCW (red) rotating dots for cued conditions. B) Adapting responses to CW (green) and CCW (red) rotating dots for uncued conditions. C) Inhibition and excitation to translation detector for cued conditions. Inhibition is sum of responses to CW and CCW rotations (i.e. red and green traces in A). Excitation arising from the translation is indicated by arrow (“Exc”) and is scaled by 1/4 to avoid overlap with inhibitory traces. D) Same as C but for uncued conditions. Inhibition is greater than for cued conditions whereas excitation is the same. E) Response of translation detector is greater for cued (left) than for uncued (right) conditions due to the greater inhibition accompanying uncued conditions. Time is in msec.

Figure 4

Feature trajectories of dots in Experiment 1 following conventions of Figures 1B and 2B. Left Column (A, C, and E): Stimuli without motion swaps. Right Column (B, D, and F): Stimuli with motion swaps. At translation onset the other dot field adopts the other dot field’s rotation direction. This manipulation reverses the relationship between cueing and motion duration for stimuli with delayed onset of one of the dot fields. We refer to stimuli with common onsets (bottom row, E and F) as the neutral condition. For stimuli with delayed onset (A-D), the interval between onsets is 750 ms. The interval between onset of the 2^nd dot field (or simultaneous onsets for E and F) and the translation is fixed at 300 ms. Following the brief translation (40 ms), both dot fields rotated for 500 ms.

Figure 5

Feature trajectories of dots in Experiment 2 following conventions of Figures 1B and 2B. Left Column (A, C, E and G): Stimuli without motion swaps. Right Column (B, D, F and H): Stimuli with motion swaps. Top Two Rows (A, B, C and D): Stimuli without color swaps. Bottom Two Rows (E, F, G and H): Stimuli with color swaps. Timing is same as in Figure 4.

Figure 6

Results of Experiment 1. Individual subject performance is shown by conjunctions of color, line style, and symbols. Mean accuracy across 11 subjects in reporting the direction of the translation in the 6 conditions of Experiment 1 is shown by bars. Each bar is the average of all subject’s performance for a given condition. Letter labels indicate stimulus conditions and correspond to those in Figure 4. For all condition types, cued trials yielded significantly better performance than uncued trials (p<0.001). Asterisks indicate significance level for pair-wise comparisons (2 and 3 asterisks corresponding to p<0.01 and p<0.001 respectively). Chance performance is 12.5% (dotted lines).

Figure 7

Performance data for the 4 conditions of Experiment 2. Data from each subject is distinguished by a unique conjunction of color, line style and symbol. Data from the two subjects that also ran in Experiment 1 are portrayed in the same way as in Figure 6 (Black upside-down triangles and Magenta asterisks). Each bar is the average of all subject’s performance for a given condition. Letter labels indicate stimulus conditions and correspond to those in Figure 5. For all condition types, cued trials yielded significantly better performance than uncued trials (p<0.001). Chance performance is 12.5% (dotted lines).

Figure 8

Schematic of hypothetical selection mechanism. A) Transparent-motion stimuli. Dark and light gray circles indicate examples of neuronal receptive fields (RFs) for areas MT and V1, respectively. The MT RF is stimulated by both dot fields, whereas the two V1 RFs are stimulated by dots from different fields. B-D illustrate 3 computational steps proposed to occur just before (B-C) and during (D) translation. Illustrated are the directional hypercolumns within areas MT (top) and V1 (bottom) that have the RFs indicated in A. Grey columns have a processing advantage. B) First computational step. Feature selective processing at a coarse spatial scale. Upward-preferring neurons in area MT have a processing advantage. This advantage might result from adaptation (see Introduction and Figure 3) or from top-down feature-specific inputs. This confers a “global” (i.e. at the scale of MT RFs) processing advantage for upward motion. C) Second processing step. Feedback from area MT onto area V1 connects neurons with similar direction preferences. Convergence between more activated feedback connections (thicker lines) and feedforward input yields an advantage for upward-preferring neurons with RFs containing upward-moving dot. This directional advantage is thus spatially restricted to dots of the cued field. D) Third processing step. Local connections spread facilitation within V1 hypercolumn. This leads to enhanced processing of any direction of motion within the collective RF of the hypercolumn.