Chad J. Marsolek

Form-Specific Visual Priming in the Right Cerebral Hemisphere

Results of 4 experiments indicate that both within-modality and case-specific visual priming for words are greater when test stimuli are presented initially to the right cerebral hemisphere (RH). In contrast, neither within-modality nor case-specific explicit memory for words is greater when stimuli are presented initially to the RH. Priming is measured using word-stem completion, and explicit memory is measured using word-stem cued recall. In both cases, Ss first rate how much they like words, and then word stems are presented briefly to the RH (in the left visual field) or to the left hemisphere (in the right visual field). Results suggest that at least 2 separate systems encode the visual representations that produce priming. The system that is more effective in the RH is better at representing form-specific information, whereas another system that is not more effective in the RH does not distinguish among distinct instances of word forms.

Dissociable Neural Subsystems Underlie Abstract and Specific Object Recognition

Participants named objects presented in the left or right visual field during a test phase, after viewing centrally presented same-exemplar objects, different-exemplar objects, and words that name objects during an initial encoding phase. In two experiments, repetition priming was exemplar-abstract yet visual when test objects were presented directly to the left cerebral hemisphere, but exemplar-specific when test objects were presented directly to the right cerebral hemisphere, contrary to predictions from single-system theories of object recognition. In two other experiments, stimulus degradation during encoding and task demands during test modulated these results in predicted ways. The results support the theory that dissociable neural subsystems operate in parallel (not in sequence) to underlie visual object recognition: An abstract-category subsystem operates more effectively than a specific-exemplar subsystem in the left hemisphere, and a specific-exemplar subsystem operates more effectively than an abstract-category subsystem in the right hemisphere.

Abstract visual-form representations in the left cerebral hemisphere.

Visual-form systems in the cerebral hemispheres were examined in 3 experiments. After learning new types of visual forms, participants rapidly classified previously unseen prototypes of the newly learned types more efficiently when the forms were presented directly to the left hemisphere (in the right visual field) than when the forms were presented directly to the right hemisphere (in the left visual field). Neither previously seen nor previously unseen distortions of the prototypes were classified more efficiently when presented directly to the left hemisphere than when presented directly to the right hemisphere. Results indicate that an abstract visual-form system operates effectively in the left hemisphere and stores information that remains relatively invariant across the specific instances of a type of form to distinguish different types. Furthermore, this system functions relatively independently of another system that operates effectively in the right hemisphere and that stores details to distinguish specific instances of a type of form.

Form-specific visual priming for new associations in the right cerebral hemisphere

In three experiments, we examined the internal processing mechanisms of relatively independent visual-form subsystems. Participants first viewed centrally presented word pairs and then completed word stems presented beneath context words in the left or right visual field. Letter-case-specific priming in stem completion was found only when the context word was the same word that had previously appeared above the primed completion word and the items were presented directly to the right cerebral hemisphere. This pattern of results was not found when participants deliberately recollected previously presented words when completing the stems. Results suggest that holistic processing, not parts-based processing as assumed in many contemporary theories of visual-form recognition, is performed in a subsystem that distinguishes specific instances in the same abstract category of form and that operates more effectively in the right hemisphere than in the left hemisphere.

Form-Specific Explicit and Implicit Memory in the Right Cerebral Hemisphere

Results from 2 divided visual field (DVF) experiments indicate that in some conditions both explicit and implicit memory are greater when same-letter-case stimuli are presented directly to the right cerebral hemisphere (in the left visual field) than when they are presented directly to the left (in the right visual field). Explicit memory was measured with word-stem cued recall, and implicit memory was measured with word-stem completion priming. Words were presented centrally during encoding, and word stems were presented directly to the right hemisphere or to the left hemisphere during testing. Results for explicit memory contrast with findings from a previous DVF study that used a different procedure, those for implicit memory replicate previous DVF findings, and both results corroborate positron emission tomography findings. We suggest that a formspecific system in the right hemisphere may contribute to both explicit and implicit memory. Research that uses positron emission tomography (PET) is currently helping to delineate areas of the human brain that play specific roles in reading and remembering words (for a review, see Petersen & Fiez, 1993). One recent discovery from PET is that a region of posterior cortex in the right cerebral hemisphere implements at least some of the processing that underlies visual repetition priming of words (Squire et al., 1992). This discovery complements recent findings from divided visual field (DVF) experiments. Under certain conditions, visual repetition priming of words is greater when test stimuli are presented directly to the right hemisphere (in the left visual field) than directly to the left hemisphere (in the right visual field; Marsolek, Kosslyn, & Squire, 1992). A second important finding from PET is that processing in the hippocampal region of the right hemisphere is involved in visual cued recall of words (Squire et al., 1992). However, findings from the DVF experiments do not corroborate this PET result. At least

Letter-Case-Specific Priming in the Right Cerebral Hemisphere with a Form-Specific Perceptual Identification Task☆☆☆★

In a form-specific perceptual identification task, subjects identify and write letter strings in the same letter case as they appear on a computer display. Letter-case-specific repetition priming was observed in this task when test items were presented directly to the right hemisphere, but not when they were presented directly to the left hemisphere, similar to results in previous word-stem completion experiments. This pattern of results was not obtained in a standard perceptual identification task. Results indicate that a specific visual-form subsystem, but not an abstract visual-form subsystem, operates more effectively in the right hemisphere than in the left, and task demands greatly affect which subsystems are recruited in different priming tests.

Viewpoint-invariant and viewpoint-dependent object recognition in dissociable neural subsystems.

Participants viewed objects in the central visual field and then named either same or different depth-orientation views of these objects presented briefly in the left or the right visual field. The different-orientation views contained either the same or a different set of parts and relations. Viewpoint-dependent priming was observed when test views were presented directly to the right hemisphere (RH), but not when test views were presented directly to the left hemisphere (LH). Moreover, this pattern of results did not depend on whether the same or a different set of parts and relations could be recovered from the different-orientation views. Results support the theory that a specific subsystem operates more effectively than an abstract subsystem in the RH and stores objects in a manner that produces viewpoint-dependent effects, whereas an abstract subsystem operates more effectively than a specific subsystem in the LH and does not store objects in a viewpoint-dependent manner.

Causality and the Allocation of Attention During Comprehension

Recent research has suggested that each statement in a narrative text is understood by relating it to its causal antecedents and consequences and that the text as a whole is understood by finding a causal path linking its opening to its final outcome. Fletcher and Bloom (1988) have proposed that in order to accomplish this goal, while minimizing the number of times long-term memory has to be searched, readers focus their attention on the last clause of a narrative that has causal antecedents but no consequences in the preceding text. As a result, a statement that is followed by a causal antecedent should remain the focus of attention, while the same statement followed by a consequence should not. This prediction was tested and confirmed in three experiments which show that when a target statement is followed by a sentence that includes only causal antecedents, (a) continuation sentences related to it are read more quickly, (b) target words drawn from it are easier to recognize, and (c) subject-generated continuations are more likely to be causally related to it.

Abstractionist versus exemplar-based theories of visual word priming: A subsystems resolution

Three experiments addressed abstractionist versus exemplar-based theories of the visual representations underlying word priming. Participants first read centrally presented whole words (each displayed in all lowercase or in all uppercase letters), and then they completed laterally presented word stems (each displayed in all lowercase or in all uppercase letters). Word stem completion priming was letter-case specific (greater for same-case primed items than for different-case primed items) when stems were presented directly to the right cerebral hemisphere but not when stems were presented directly to the left cerebral hemisphere. This interaction was not influenced by the typicality of the test stems, but it was observed only for stems composed of letters with visually dissimilar lowercase and uppercase structures (e.g., bea/BEA) and not for stems composed of letters with visually similar lowercase and uppercase structures (e.g., sco/SCO). In contrast, cued recall was letter-case specific when similar-case or dissimilar-case stems were presented directly to the right hemisphere. Results do not support strongly abstractionist or exemplar-based theories. Instead, they suggest a resolution to these differing perspectives: Relatively independent neural subsystems operate in parallel to underlie abstract-category and specific-exemplar priming of word forms.

What form of memory underlies novelty preferences

Novelty preferences (longer fixations on new stimuli than on previously presented stimuli) are widely used to assess memory in nonverbal populations, such as human infants and experimental animals, yet important questions remain about the nature of the processes that underlie them. We used a classical conditioning paradigm to test whether novelty preferences reflect (1) a stimulus-driven bias toward novelty in visual selective attention or (2) explicit memory for old stimuli. Results indicated that conditioning affected adults’ looking behavior in the visual paired comparison, but not their recognition memory judgments. Furthermore, the typically observed novelty preference occurred only when a bias toward novelty had no competition from a bias toward salience due to conditioning. These results suggest that novelty preferences may reflect attentional processes and implicit memory to a greater degree than explicit memory, a finding with important implications for understanding memory in nonverbal populations and the development of memory in humans.