Individual differences in stroop and local-global processing: a possible role of interhemispheric interaction.

Two experiments are reported examining individual differences in the processing of centrally presented stimuli containing two dimensions of information lateralized to opposite cerebral hemispheres. Left-handers, arising from (a) their lesser degree of functional lateralization and (b) their greater degree of callosal connectivity, were hypothesized to exhibit greater interdimensional (and presumably interhemispheric) interaction. Experiment 1 utilized local-global stimuli, and left-handers were found to be impaired at keeping the two dimensions independent and superior at integrating the two dimensions. Experiment 2 used Stroop stimuli, and left-handers again were impaired at keeping the two dimensions independent (i.e., showed greater Stroop interference). Correlational analyses indicated that the mechanisms of interdimensional integration versus independence are at least partially independent from one another. Results suggest that aspects of interhemispheric interaction can be addressed via the use of nonlateralized input.

Superior episodic memory is associated with interhemispheric processing.

The dependence of episodic memories on interhemispheric processing was tested. In Experiment 1, positive familial sinistrality (FS+; e.g., presence of left-handed relatives) was associated with superior episodic memory and inferior implicit memory in comparison with negative familial sinistrality (i.e., FS-). This reflected a greater degree of interhemispheric interaction in FS+ participants, which was hypothesized as facilitating episodic memory. In Experiment 2, the authors directly manipulated inter- versus intrahemispheric processing using tests of episodic (recognition) and semantic (lexical decision) memory in which letter strings were presented twice within trial blocks. Semantic memory was superior when the 2nd presentation went to the same hemisphere as the 1st. Episodic memory, however, was superior when the 2nd presentation of a stimulus went to the opposite hemisphere. Results support an interhemispheric processing basis for episodic memories.

Visual field differences in spatial frequency discrimination.

Subjects discriminated between sine-wave gratings that differed by either +/-0.125 octaves (small difference) or +/-1.0 octaves (large difference). Baseline stimuli consisted of either 1.0 or 4.0 cycles per degree gratings. A left visual field advantage was obtained for the small difference in frequency, with no visual field advantages for the large difference in frequency. Similarly, moderate support for right versus left visual field advantages in processing high versus low spatial frequencies was found, although these interactions were not statistically significant. The results are discussed in light of Kosslyn's (1987) categorical and coordinate framework.

Hemispheric asymmetries in the identification of band-pass filtered letters.

Processing of band-pass filtered letters in the left versus right cerebral hemispheres (LH vs. RH) was examined. The present experiments constituted a partial replication of a study in which Peterzell, Harvey, and Hardyck (1989) found no hemispheric differences in accuracy or reaction time (RT) as a function of spatial frequency. However, methodological limitations of their study (e.g., the possibility that subjects were engaged in a detection, not identification, task) may have obscured possible hemispheric differences. We addressed these problems in the present study, obtaining significant hemisphere × spatial frequency interactions for RT andd', with RH advantages at low frequencies and LH advantages at high frequencies; however, these effects were not large in magnitude and were often restricted to particular dependent variables, stimulus sizes, and so forth. Hemispheric differences in response bias were also found.

Questions of criteria: Reply to Peterzell (1997).

In a reply to our report on hemispheric differences in processing band-pass filtered letters (Christman, Kitterle, & Niebauer, 1997), Peterzell (1997) argues that our results are not attributable to hemispheric asymmetries in spatial frequency processing. Rather, Peterzell argues that factors such as response criteria and stimulus visibility can account for our results. We argue that our results are attributable (at least in part) to hemispheric asymmetries in spatial frequency processing, while at the same time we acknowledge the potential influence of other factors in the determination of hemispheric differences.

Hemispheric processing of form versus texture at the local level of hierarchical patterns.

Kimchi (Kimchi, R., 1992. Primacy of wholistic processing and global/local paradigm: A critical review. Psychological Bulletin 112, 24-38) proposed that the perception of hierarchical stimuli is dependent on the number of local elements. The local level of stimuli with smaller numbers of elements is perceived as discrete forms, and irrelevant form information at the global level affects processing of the local level; with larger numbers of local elements, however, the local level is seen as being comprised of a texture, and interlevel interaction is eliminated. The current study provides a test of Kimchi's hypothesis for left versus right cerebral hemispheric (LH vs. RH) processing, employing a stimulus set that sampled the critical range of a number of local elements more thoroughly than previous studies. Results indicate that (i) increasing the number of local elements reduces and eventually eliminates interlevel interference, (ii) the crossover point between perception of local elements as form vs. texture does not differ for LH vs. RH processing, and (iii) hemispheric differences in local-global processing of stimuli comprised of geometric shapes are not robust, although an LH advantage for local targets was obtained for stimuli with few local elements.

Equivalent perceptual asymmetries for free viewing of positive and negative emotional expressions in chimeric faces.

Research employing chimeric stimuli (in which smiling and neutral half-faces are paired) has demonstrated greater influence of the left half-face in determining perceived intensity of expression. To date, no studies have examined how emotional expressions other than happiness are perceived in this format. Right-handed subjects viewed chimeric faces depicting both positive (happiness, pleasant surprise) and negative (sadness, anger) emotions in a free vision task. Results indicated a left half-face bias for all four emotions, supporting the hypothesis of a greater right hemisphere role in emotional perception. The lack of differences in strength of left half-face bias as a function of the specific emotion depicted suggests that results obtained with typical chimeric half-face paradigms can be generalized to emotions other than happiness.

On the complex relation between perceptual characteristics and hemispheric asymmetry.

The relation between hemispheric asymmetry and perceptual characteristics in general, and the spatial frequency content of input in particular, are discussed in reference to Peterzell's (1991) reply to a literature review by Christman (1989a). Three main points are presented: (1) there is evidence that spatial frequency, independent of the total information/energy content of input, does influence patterns of hemispheric asymmetry; (2) that information and energy represent potentially dissociable constructs that need to be considered separately; and (3) that there is recent evidence that numerous input and task characteristics, other than spatial frequency content, influence hemispheric asymmetry (e.g., spatial phase, relative spatial frequency, temporal frequency, task requirements, etc.).