Unconscious processing dissociates along categorical lines

Abstract

Visual object recognition is subserved by ventral temporal and occipital regions of the brain. Regions comprising the dorsal visual pathway have not been considered relevant for object recognition, despite strong categorical biases for tool-related information in those regions. Here, we show that dorsal stream processes influence object categorization. We used two techniques to render prime pictures invisible: continuous flash suppression (CFS), which obliterates input into ventral temporal regions, but leaves dorsal stream processes largely unaffected, and backward masking (BM), which allows suppressed information to reach both ventral and dorsal stream structures. Categorically congruent primes suppressed under CFS facilitate categorization of tools but have no effect on nonmanipulable objects; in contrast, primes rendered invisible through BM facilitate target categorization for both tools and nonmanipulable things. Our findings demonstrate that information computed by the dorsal stream is used in object categorization, but only for a category of manipulable objects.

Fig. 1.

Stimuli rendered invisible with CFS. For experiments 1–5, we used CFS to render the prime stimuli invisible. In CFS a static image competes with a dynamic image, with the latter reliably suppressing the former for a prolonged time (17, 18). Low-luminance, variable low-contrast versions of the prime stimuli were created. Two different high-contrast random noise patterns were created per prime. To ensure that prime stimuli and high-contrast patterns were presented to separate eyes, prime stimuli were restricted to the green RGB channel, whereas high-contrast random patterns were restricted to the red RGB channel. Red/green anaglyph glasses were worn by participants throughout the experiments; it was ensured that the red lens (and hence the high-contrast noise pattern) corresponded to the dominant eye of each participant (the Miles test was used to determine eye dominance). Following previous studies on CFS (17, 18), we presented each random pattern for 100 ms (10 hz dynamic noise patterns), for a total prime/random noise (composite) presentation of 200 ms.

Fig. 2.

Experimental design. The experiments consisted of two independent stages, the experiment proper, and a detection (experiment 1) or discrimination (experiments 2–6) task performed immediately after the experiment proper. The procedure for the detection and discrimination tasks was the same as in the experiment proper except that, first, the target picture was not presented; second, participants were fully informed of the presence of a prime stimulus; and third, participants were instructed to perform the task (detection or discrimination) over the prime stimulus. The trial structure for the experiments was the following: (A) for experiments 1–5 a fixation cross appeared on the screen (500 ms), followed immediately by the prime picture accompanied by the first random noise pattern (100 ms), followed immediately by the prime picture accompanied by the second random noise pattern (100 ms), followed by the target picture (3,000 ms or response, whichever came first); (B) in experiment 6, the trial sequence was the following: a fixation cross appeared on the screen (500 ms), immediately followed by the prime picture (35 ms), immediately followed by a black and white random noise mask (100 ms), immediately followed by the target picture (3,000 ms or until the response of participants, whichever came first).

Fig. 3.

Behavioral priming effects. Average priming effects (incongruent trials minus congruent trials) plotted as a function of the experimental conditions. *, P < 0.05; **, P < 0.001. Error bars represent SEM for priming effects across subjects.