Emmorey, K., Hickok, G., & Klima, E. S. (1995). Differences in mental rotation within linguistic and nonlinguistic domains: Evidence from an ASL signer with right hemisphere damage. Brain and Language, 51(1), 176-179.
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American Sign Language (ASL) uses space itself to encode spatial information in an isomorphic fashion. For most locative expressions in ASL, there is a direct correspondence between the location of the hands in signing space and the position of physical objects in the real world. These spatial scenes are most often described from the perspective of the narrator, such that the viewer, if facing the signer, must perform a 180¡ mental rotation to correctly comprehend the description (see Fig. 1a). But ASL also has a marked sign glossed as YOU-ENTER which signals that the description is signed from the addresseeÕs perspective, thus obviating the need for a 180¡ mental rotation (see Fig. 1b). It is well-known that tasks requiring mental rotation are more demanding than tasks which do not. But given that the canonical form of locative descriptions in ASL requires mental rotation, the following questions arise. (1) Does this difficulty with rotation hold for processing ASL? (2) Is mental rotation ability for spatial representations within a linguistic domain dissociable from nonlinguistic mental rotation? We present case AM, a right hemisphere damaged ASL signer, who exhibits a dissociation between her ability to mentally rotate a sequentially presented array of objects vs. a signed description of those objects, with the hands positioned in space to indicate object location. We also show that normal ASL signers show a strong reversal effect for stimuli involving real objects, but not for signed descriptions of the location of these objects -- despite the transparency between the linguistic and nonlinguistic presentations of object location.
Patient Profile. AM is an eighty year-old deaf woman who suffered a right hemisphere stroke three years prior to testing. AM became deaf at age two, attended a residential school for the deaf, married a deaf man, and prefers ASL as her primary means of communication. She is not aphasic for ASL and performed well on the Sign Diagnostic Aphasia Exam (based on the BDAE). However, she exhibited some moderate visuospatial problems: she showed mild left neglect and performed poorly on the WAIS blocks task, occasionally breaking the configuration. AM also performed particularly poorly (3/50) on a mental rotation task which requires the subject to decide whether two forms are the same or mirror images, regardless of orientation. Emmorey, Kosslyn, and Bellugi (1993) have shown that ASL signers exhibit superior performance on this task and argue that the enhancement is language-linked. Given AMÕs poor performance on this task, we wanted to compare her mental rotation abilities with ASL and with objects, by closely controlling the similarities between the two domains.
Method. AM and eight normal deaf ASL signers (mean age = 24) viewed videotapes of furniture (a chair, a bed, a dresser, or a table) appearing briefly and sequentially on a board marked with an entrance which either matched the entrance on an identical board in front of them or was rotated 180¡ (See Fig. 1). Subjects were asked to place furniture on their board in the orientation and location shown on the video, making the appropriate rotation when required. Subjects then viewed a video in which the same scenes were signed from the two perspectives, i.e., rotation required (I- ENTER) or no rotation required (YOU-ENTER). The appearance of the furniture on the board was timed to correspond to the length of time required to articulate the sign and position the classifier form in space during the Sign condition. There were three practice items for each condition. Half of the trials required a rotation and half did not, and these were mixed (not blocked). The number of stimuli ranged from two to four items, four trials each, resulting in 12 trials and 18 object placements per condition. The responses were exactly the same for the Sign and Object conditions.
Results. We tabulated the number of omission, location, and orientation errors in each condition. Fig. 1c summarizes the results collapsed over error type. Normal Subjects: Performance in the Object condition was consistent with previous findings on mental rotation: greater accuracy in the no rotation condition compared to the rotation condition. In the Sign condition, the typical effect of rotation was not found: performance in the no rotation condition (the YOU-ENTER stimuli) was actually worse than in the rotation condition (the I-ENTER stimuli). Excluding omission errors where normals were at ceiling, accuracy decreased by 16% when rotation was required for the object stimuli, but accuracy increased 10% for the sign stimuli. This pattern was true for both location and orientation errors. Case AM: AMÕs performance in the Object condition revealed a large effect of rotation: Her performance in the no rotation condition was near normal, but in the rotation condition she had far more errors than normal subjects (> 2 s.d. below the mean for each error type). In fact, AM did not correctly locate or orient any of the objects she placed. Clearly, her mental rotation ability is severely restricted. AMÕs performance in the Sign condition, however, differed sharply from her performance in the object condition. Like normal subjects, AM did not show a typical effect of rotation: she made fewer errors in the rotation condition. Furthermore, AM performed substantially better in the Sign rotation condition than in the Object rotation condition. The fact that AM had little difficulty performing mental rotation on signed stimuli is surprising given her significant deficit in mentally rotating object stimuli.
Conclusions. These results indicate that ASL signers treat signing space differently than they do nonsigning space, such that the effect of mental rotation is attenuated or reversed for signs located in physical space compared to objects located in a similar space. In addition, right hemisphere damage may produce a greater impairment for mental rotation of real objects, suggesting that mental rotation for a linguistic signal may be more insulated from the effects of right parietal damage.
Reference:
Emmorey, K., Kosslyn, S. M., & Bellugi, U. (1993). Visual imagery and visual- spatial language: Enhanced imagery abilities in deaf and hearing ASL signers, Cognition, 46, 139 - 181.
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