Asymmetries in facial recognition: evidence for a memory component

Memory search for faces and digits in patients with unilateral brain lesions

To clarify whether the impairment of right-brain-damaged (RBD) patients in face recognition is related to perceptual or mnestic processing stages, we tested unilaterally lesioned patients and controls in a Sternberg-type memory search task. Subjects had to memorize sets of 1, 2, or 3 faces (or, in a control condition, digits) and were then to recognize these memorized stimuli among new ones by speeded choice reactions. In this task, deficits in stimulus encoding and memory search should show up in increased intercepts and slopes, respectively, of the RT function over memory set size. A face-specific impairment of the RBD patient group, consisting in longer reaction times and higher error rates, was confirmed but could not be unequivocally attributed to either stimulus encoding or memory search. However, inspection of individual data suggested that (1) some RBD patients are virtually unimpaired in face recognition and (2) if impairment after right hemisphere damage is present, it may selectively affect either stimulus encoding or memory search.

Contributions of stimulus encoding and memory search to right hemisphere superiority in face recognition: behavioural and electrophysiological evidence

The relative contributions of stimulus encoding and memory search to right hemisphere (RH) superiority in face recognition were investigated in a memory search task with lateralized probe stimulus presentation. RH superiority in reaction time and latency of a late positive ERP component was independent of stimulus degradations, but increased as a function of memory set size. These results suggest equal efficiency of the hemispheres in stimulus encoding, but RH superiority in memory search. In addition, a long-lasting ERP negativity contralateral to the stimulated hemifield is discussed with reference to models of hemispheric processing in hemifield studies.

Right posterior brain-damaged patients are poor at assessing the age of a face

The ability to order unknown faces by age was investigated in right and left brain-damaged patients, divided into posterior and non-posterior groups on the basis of CT scan findings. A face recognition test and a figure ground discrimination test were also given. All three tests were affected by brain damage, but their sensitivity to the locus and side of lesion varied. While no hemispheric difference was found on the figure ground discrimination test, the face age test significantly discriminated patients with right posterior injury from any other brain-damaged group. The face recognition test occupied an intermediate position, with right posterior patients significantly impaired in comparison with right non-posterior patients and marginally impaired with respect to left posterior patients. Aphasia did not affect the performance of left brain-damaged patients on any of the tests. The findings are interpreted as evidence that damage of the right posterior hemisphere areas disrupts the structural encoding of visual information. Four prosopagnosic patients were also tested. Only those showing signs of apperceptive agnosia failed on the face age test.

Familiarity, spatial frequency and task determinants in processing laterally presented representations of faces

Laterally presented portraits were judged as familiar or unfamiliar (Experiments 1 and 2), or were processed in a target matching task (Experiment 3). Exposure durations (100 or 190 msec.) were varied between (Experiments 1 and 2) or within (Experiment 3) the experiments. Clear and degraded faces appeared either randomly intermingled (Experiment 1) or in blocked sequences (Experiments 2 and 3). In Experiment 3 familiar and unfamiliar stimuli also appeared in blocks. Stimulus degradation was achieved by coarse quantization of the image into blocks, so replacing relevant high-spatial-frequencies (the features) by spurious information, preserving only the original lower-spatial-frequency components. Left hemisphere mediation was strongest and most consistent with long exposures and random sequences of clear and degraded stimuli. Right hemisphere mediation tended to appear with shorter exposures and degraded stimuli presented in blocks. Though the interactions were often complex, the general pattern of results was consistent with the analytic-holistic processing dichotomy.

Familiarity decisions for faces presented to the left and right cerebral hemispheres

Right-handed subjects were asked to decide whether or not faces presented briefly in the RVF or in the LVF were familiar (familiar faces were those of famous people). This task avoids the need for extensive semantic processing or temporary storage involved in conventional naming or identification tasks, and thus eliminates the contribution of such factors to any observed asymmetry. The resulting finding of faster reaction times to LVF faces, with no overall visual hemifield difference in error rates, is taken to indicate a right-hemisphere superiority either in the processes used to construct facial representations or in the accessing of face recognition units, or both.

Right cerebral hemisphere superiority for constructing facial representations

Right-handed people were asked to decide whether or not stimuli presented in the left visual hemifield (LVF) or in the right visual hemifield (RVF) were faces. Manual reaction times and error rates were recorded under three conditions. In Condition A, stimuli were line drawings of faces and moderately scrambled nonfaces made by rearranging the facial features. In Condition B, stimuli were line drawings of faces and highly scrambled nonfaces. In Condition C, stimuli were line drawings of faces and objects. The results show that faces are identified more quickly from the LVF than from the RVF in Condition A (faces vs moderately scrambled nonfaces), with no visual hemifield difference in reaction times to Condition B (faces vs highly scrambled nonfaces) and Condition C (faces vs objects). These findings are taken to indicate that both cerebral hemispheres are able to assign stimuli to the "face" category, but the right hemisphere is better than the left hemisphere at constructing facial representations. This cerebral hemisphere difference in ability to construct facial representations becomes evident when a detailed representation is required (as in Condition A).

Hemispheric asymmetry in memory search for four-letter names and human faces

Two memory search experiments were conducted using vertically oriented four-letter names and human faces as stimuli. Subjects were required to indicate as quickly and as accurately as possible whether or not a single probe stimulus (presented for 150 msec to either the left or right visual field) was contained in a set of 2, 3, 4, or 5 items being held in short-term memory. The probe stimuli were presented alone (clear condition) or centrally embedded in a matrix of dots (degraded condition). In Experiment 1 (involving names), a right visual field/left hemisphere advantage was obtained and pinpointed at the encoding stage rather than at the memory comparison stage of the information-processing system. For Experiment 2 (involving human faces), no hemispheric advantage was readily observed. In each experiment, both the left hemisphere and the right hemisphere employed an abstract memory comparison operation from which the effects of probe degradation have been removed. These results are discussed in terms of their implications for various models of hemispheric asymmetry.

Cerebral asymmetries in processing proper names: evidence of an efficiency difference

An investigation of the capacities of the cerebral hemispheres to process proper names was conducted using a memory-scanning paradigm. The results indicated that rate at which the test name was compared against the memory set items varied with the visual half-field stimulated. This is interpreted as indicating that both hemispheres employed the same comparison process but that the left hemisphere was more efficient, reflecting a quantitative hemisphere asymmetry.