Susan M. Ravizza

Functional dissociations within the inferior parietal cortex in verbal working memory

Neuroimaging studies of working memory have revealed two sites in the left supramarginal gyrus that may support the short-term storage of phonological information. Activation in the left dorsal aspect of the inferior parietal cortex (DIPC) has been observed in contrasts of working memory load, whereas activation in the ventral aspect of the inferior parietal cortex (VIPC) has been found primarily in contrast of information type (verbal vs. nonverbal). Our goal was to determine whether these two areas are functionally distinct or if instead they are part of a homogeneous region with large variations in the focus of peak activity. Toward this end, we used fMRI to assess the neural response in two working memory tasks (N-back and item recognition) in which we also manipulated memory load and the type of information to be recalled (verbal vs. nonverbal). We found both DIPC and VIPC activation in the same group of subjects and further demonstrated that they have differential sensitivity to our experimental factors. Only the DIPC showed robust load effects, whereas only the VIPC showed reliable effects of information type. These results help to account for the differences observed in between-subject comparisons, and they indicate that the two regions are functionally dissociable. In contrast to the DIPC, activity of the VIPC was also recruited in the fixation and low-load conditions, a surprising result that has not been fully explored in prior studies. Despite their distinctive patterns of performance, neither of these regions displayed a pattern of activity that entirely corresponds to common assumptions of a dedicated phonological short-term store (STS). Instead, we hypothesize that the DIPC may support domain-general executive processes, while the VIPC may support phonological encoding-recoding processes central to a variety of language tasks.

The Neural Substrates of Cognitive Control Deficits in Autism Spectrum Disorders

Executive function deficits are among the most frequently reported symptoms of autism spectrum disorders (ASDs), however, there have been few functional magnetic resonance imaging (fMRI) studies that investigate the neural substrates of executive function deficits in ASDs, and only one in adolescents. The current study examined cognitive control - the ability to maintain task context online to support adaptive functioning in the face of response competition - in 22 adolescents aged 12-18 with autism spectrum disorders and 23 age, gender, and IQ matched typically developing subjects. During the cue phase of the task, where subjects must maintain information online to overcome a prepotent response tendency, typically developing subjects recruited significantly more anterior frontal (BA 10), parietal (BA 7 and BA 40), and occipital regions (BA 18) for high control trials (25% of trials) versus low control trials (75% of trials). Both groups showed similar activation for low control cues, however the ASD group exhibited significantly less activation for high control cues. Functional connectivity analysis using time series correlation, factor analysis, and beta series correlation methods provided convergent evidence that the ASD group exhibited lower levels of functional connectivity and less network integration between frontal, parietal, and occipital regions. In the typically developing group, fronto-parietal connectivity was related to lower error rates on high control trials. In the autism group, reduced fronto-parietal connectivity was related to attention deficit hyperactivity disorder symptoms.

A functional and structural study of emotion and face processing in children with autism

Children with autism exhibit impairment in the processing of socioemotional information. The amygdala, a core structure centrally involved in socioemotional functioning, has been implicated in the neuropathology of autism. We collected structural and functional magnetic resonance images (MRI) in children 8 to 12 years of age with high-functioning autism (n=12) and typical development (n=15). The functional MRI experiment involved matching facial expressions and people. Volumetric analysis of the amygdala was also performed. The results showed that children with autism exhibited intact emotion matching, while showing diminished activation of the fusiform gyrus (FG) and the amygdala. Conversely, the autism group showed deficits in person matching amidst some FG and variable amygdala activation. No significant between-group differences in the volume of the left or right amygdala were found. There were associations between age, social anxiety and amygdala volume in the children with autism such that smaller volumes were generally associated with more anxiety and younger age. In summary, the data are consistent with abnormalities in circuits involved in emotion and face processing reported in studies of older subjects with autism showing reductions in amygdala activation related to emotion processing and reduced fusiform activation involved in face processing.

Using neuroimaging to evaluate models of working memory and their implications for language processing

Abstract The precise relationships that exist between verbal working memory and language processing remain underdetermined. We argue that while neuroimaging methods have the potential to provide important insights into the links between these two cognitive domains, an over-reliance on a single theoretical perspective has stunted progress toward this end. Specifically, neuroimaging studies in the domain of working memory have relied heavily on the theoretical framework provided by Baddeleys Multiple-Component Model. We reexamine the sufficiency of this model in accounting for a range of neuroimaging evidence, and draw attention to a set of findings that are not readily explained by a conventional mapping of this model onto specific neuroanatomical substrates. An alternative framework provided by Cowans Embedded-Processes Model is then evaluated, and we conclude that this alternative view supports a reconceptualization of the contributions of Brocas area and the left inferior parietal cortex to working memory that can account for a wide range of findings. Importantly, this alternative account suggests links between working memory and language that are not afforded by currently prevailing interpretations.

Comparison of the Basal Ganglia and Cerebellum in Shifting Attention

The basal ganglia and cerebellum have traditionally been associated with motor performance. Recently, there has been considerable interest regarding the contributions of these subcortical structures to aspecdts of cognition. In particular, both the basal ganglia and cerebellum have been hypothesized to be involved in the control of attentional set. To dat, no neuropsychological studies have directly compared the effects of basal ganglia and cerebellar dysfunction on the same attention shifting tasks. To this end, dwe employed and alternating attention task that has been used to demonstrate putative attentional control deficits in children with cerebellar pathology, either related to autism or neurological insult. When adult patients with either Parkinsons disease or cerebellar lesions were tested on this task, a similar pattern of deficits was observed for both groups. However, when the motor demands were reduced, cerebellar patients showed a significant improvement on the alternating attention task, whereas the Parkinson patients continued to exhibit an impairment. This dissociation suggests that attentional deficits reported previously as being due to cerebellar dysfunction may be, at least in part, secondary to problem related to coordinating successive responses. In contrast, attention-shifting deficits associated with basal ganglia impairment cannot be explained by recourse to the motor demands of the task.

Shifting set about task switching: behavioral and neural evidence for distinct forms of cognitive flexibility.

Task switching is an important aspect of cognitive control and understanding its underlying mechanisms is the focus of considerable research. In this paper, we contrast two different kinds of task switching paradigms and provide evidence that different cognitive mechanisms underlie switching behavior depending on whether the switch is between sets of rules (rule switch) or sets of features presented simultaneously (perceptual switch). In two experiments, we demonstrate that behavioral effects (Experiment 1) and neural recruitment (Experiment 2) vary with the type of switch performed. While perceptual switch costs occurred when the alternative feature set was physically present, rule switch costs were observed even in their absence. Rule switching was also characterized by larger target repetition effects and by greater engagement of the dorsolateral prefrontal cortex. In contrast, perceptual switching was associated with greater recruitment of the parietal cortex. These results provide strong evidence for multiple forms of switching and suggest the limitations of generalizing results across shift types.

Contributions of the Prefrontal Cortex and Basal Ganglia to Set Shifting

Impairments of set shifting have been associated with damage to both the prefrontal cortex (PFC) and to the basal ganglia. The purpose of these experiments was to determine whether damage to the PFC was associated with shifting impairments per se or whether any switching deficits could be attributed to a reduction of working memory capacity. In contrast, shifting impairments were expected for Parkinson patients regardless of memory load, given that these patients seem to have no cognitive deficits other than when having to shift set. To vary working memory demands, a cue to the relevant dimension (letter or shape) in an odd-man-out task was presented or withheld. Pathology to prefrontal areas associated with normal aging was not linked to shifting deficits when working memory load was reduced in a comparison of older and younger adults (Experiment 1). In contrast, set-shifting abilities were still impaired for stroke patients with prefrontal damage regardless of working memory demands (Experiment 2). Parkinson patients were relatively unimpaired on this task (Experiment 2), but began to display shifting deficits when response competition was present in the display (Experiment 3).

Reduced phonological similarity effects in patients with damage to the cerebellum.

Ten cerebellar patients were compared to 10 control subjects on a verbal working memory task in which the phonological similarity of the words to be remembered and their modality of presentation were manipulated. Cerebellar patients demonstrated a reduction of the phonological similarity effect relative to controls. Further, this reduction did not depend systematically upon the presentation modality. These results first document that qualitative differences in verbal working memory may be observed following cerebellar damage, indicating altered cognitive processing, even though behavioral output as measured by the digit span may be within normal limits. However, the results also present problems for the hypothesis that the cerebellar role is specifically associated with articulatory rehearsal as conceptualized in the Baddeley-Hitch model of working memory.

Left TPJ activity in verbal working memory: Implications for storage- and sensory-specific models of short term memory

Patients with damage to the left temporoparietal junction (TPJ) have a low verbal span without concomitant deficits in speech perception. This pattern of cognitive impairment is taken as evidence for a dedicated phonological buffer that plays little role in perception (storage-specific account). In contrast, other research suggests that items are maintained and perceived in the same regions (sensory-specific account). In an fMRI study, we demonstrate that the left TPJ does not respond in a way predicted of a phonological buffer; that is, activity in this region is not sustained during encoding or maintenance. Instead, a region in the superior temporal gyrus that has been associated with both speech perception and production demonstrated the expected profile of a store: it was more active in the verbal condition than the object condition and was active during both encoding and maintenance. These results support the sensory-specific account of short term memory rather than the storage-specific account. Based on the pattern of activity in the left TPJ, we suggest that the impairment of verbal working memory observed in patients with TPJ damage may be due to diminished attentional processes rather than reduced storage capacity.

Movement and lexical access: Do noniconic gestures aid in retrieval?

The production of meaningful gestures has been claimed to enhance lexical access. However, the possibility that meaningless movements also improve retrieval has been largely ignored despite evidence that all types of movements increase with dysfluency. To examine this issue, we conducted two experiments to determine whether movements in general would improve lexical access in a tip–of–thetongue (TOT) paradigm. TOT states were induced by presenting definitions of rare words that participants were then asked to recall. Participants who were required to tap at their own pace while retrieving words obtained significantly higher resolution rates than those who were immobile. Thus, movement does not have to be semantically related to the lexical item in order to aid in retrieval. However, tapping did not improve lexical access in all retrieval tasks. In a lexical retrieval task that relied more on executive abilities (letter fluency), participants who tapped retrieved fewer words than those who were immobile. The fact that movement enhanced lexical access only when retrieval depended on the automatic spread of activation suggests that facilitation may occur because of the activation of neural areas common to both speech and movement.