Differences between visual hemifields in identifying rapidly presented target stimuli: letters and digits, faces, and shapes

Abstract

The right hemisphere has been shown to play a dominant role in processing of visuo-spatial information. Recently, this role has been studied in the two-stream rapid serial visual presentation task. In this task, two alphanumerical targets are embedded in left and right simultaneous streams of rapidly changing letters. The second target (T2) is identified better in the left than in the right visual field. This difference has been interpreted as advantage of the right hemisphere (RH). However, a disadvantage of the left hemisphere (LH) could not be excluded so far. The LH, specialized for processing of verbal stimuli, might be overloaded due to constant input of letters from both visual fields. In the present study, this overload hypothesis was tested by reducing demands on verbal processing (Experiment 1), and by overloading the RH with non-verbal stimuli: faces (Experiment 2) and irregular shapes (Experiment 3). The left visual field advantage proved to be largely independent from the level of verbal load and from stimulus type. Therefore, although not entirely disproving the overload hypothesis, these results suggest as the most parsimonious explanation this asymmetry reflects a RH advantage, presumably in perceptual and attentional processing, rather than a LH disadvantage caused by verbal overload.

Keywords: RSVP; hemispheric asymmetry; hemispheric specialization; lateralization; left visual-field advantage; visual perception.

Figure 1

An example of stimuli and sequence of events in a trial with standard number of distractors (A), and with reduced number of distractors (B). See Methods for details. Red color is replaced here by white.

Figure 2

Identification rates of T1 and T2 in Experiment 1. Results are displayed for T1 in the upper left panel, for T2 from the dual-target task in the lower left panel, and for T2 from the single-target task in the lower right panel.

Figure 3

An example of the face-stimuli used in Experiment 2 as T1 (A) and T2 (B). Pictures were taken from the NimStim Set of Facial Expression (Tottenham et al., ; http://www.macbrain.org/resources.htm).

Figure 4

Rates of T1 and T2 identification in Experiments 2A (left panel) and 2B (right panel). T1 identification rates were calculated from all trials, and T2 identification rates were computed from all correctly identified T1 trials.

Figure 5

Stimuli used in Experiment 3 as T1 (A), T2 (B), and distractors (C). Red color is replaced here by gray.

Figure 6

Rates of T1 and T2 identification in Experiment 3. T1 identification rates were calculated from all trials, and T2 identification rates were computed from all correctly identified T1 trials.