Irina M. Harris

Parietal Lobe Contribution to Mental Rotation Demonstrated with rTMS

A large number of imaging studies have identified a role for the posterior parietal lobe, in particular Brodmanns area 7 and the intraparietal sulcus (IPS), in mental rotation. Here we investigated whether neural activity in the posterior parietal lobe is essential for successful mental rotation performance by observing the effects of interrupting this activity during the execution of a mental rotation task. Repetitive transcranial magnetic stimulation (rTMS) was applied to posterior parietal locations estimated to overlie Brodmanns area 7 in the right and the left hemisphere, or to a posterior midline location (sham condition). In three separate experiments, rTMS (four pulses, 20 Hz) was delivered at these locations either 200400, 400600, or 600800 msec after the onset of a mental rotation trial. Disrupting neural activity in the right parietal lobe interfered with task performance, but only when rTMS was delivered 400 to 600 msec after stimulus onset. Stimulation of the left parietal lobe did not reliably affect mental rotation performance at any of the time points investigated. The time-limited effect of rTMS was replicated in a fourth experiment that directly compared the effects of rTMS applied to the right parietal lobe either 200400 or 400600 msec into the mental rotation trial. The results indicate that the right superior posterior parietal lobe plays an essential role in mental rotation, consistent with its involvement in a variety of visuospatial and visuomotor transformations.

Mental-Rotation Deficits Following Damage to the Right Basal Ganglia

The authors report the case of a woman with a right basal ganglia lesion and severe mental-rotation impairments. She had no difficulty recognizing rotated objects and had intact left-right orientation in egocentric space but was unable to map the left and right sides of external objects to her egocentric reference frame. This study indicates that the right basal ganglia may be critical components in a cortico-subcortical network involved in mental rotation. We speculate that the role of these structures is to select and maintain an appropriate motor program for performing smooth and accurate rotation. The results also have important implications for theories of object recognition by demonstrating that recognition of rotated objects can be achieved without mental rotation.

When more is less: extraction of summary statistics benefits from larger sets.

Despite several processing limitations that have been identified in the visual system, research shows that statistical information about a set of objects could be perceived as accurately as the information about a single object. It has been suggested that extraction of summary statistics represents a different mode of visual processing, which employs a parallel mechanism free of capacity limitations. Here, we demonstrate, using reaction time measures, that increasing the number of stimuli in the set results in faster reaction times and better accuracy for estimating the mean tendency of a set. These results provide clear evidence that extraction of summary statistics relies on a distributed attention mode that operates across the whole display at once and that this process benefits from larger samples across which the summary statistics are calculated.

Object Orientation Agnosia: A Failure to Find the Axis?

A dissociation between the ability to recognize misoriented objects and to determine their orientation has been reported in a small number of patients with vascular lesions. In this article, we describe a 57-year-old man with probable Alzheimers disease who shows the same dissociation. Neuro-imaging findings indicated marked hypometabolism in the posterior cortical regions, particularly the postero-superior parietal lobes. Clinically, the patient had good object recognition accompanied by severely impaired spatial abilities. The experimental investigations comprised a variety of tasks in which he identified misoriented objects, evaluated the orientation of single objects, or discriminated the orientation of simultaneously presented items. Results revealed that his object recognition was independent of orientation and was largely mediated by salient features. With respect to orientation judgements, the patient displayed a profound inability to judge the orientation of nonupright objects, but remarkably intact (though largely implicit) knowledge of the upright orientation. Strikingly, his orientation judgements were also more accurate for upside-down objects than for other orientations (i.e., 90). We interpret these results as evidence that judgements about object orientation are facilitated when the orientation of the principal axis of the object matches that of an internal representation. We propose that the inability to determine other orientations may be due to the failure of an axis-finding mechanism implemented in the posterior parietal lobes, that translates between object-centered and eye-centered coordinates appropriate for guiding visual scanning.

Effects of right parietal transcranial magnetic stimulation on object identification and orientation judgments

We investigated the role played by the right parietal lobe in object identification and the ability to interpret object orientation, using transcranial magnetic stimulation (TMS) to momentarily interfere with ongoing cortical activity. Short trains of TMS pulses (12 Hz) were applied to a site overlying the right intraparietal sulcus/inferior parietal lobe while subjects performed either object identification tasks (i.e., picture-word verification and categorizing objects as natural or manufactured) or object orientation judgment tasks (i.e., picture-arrow verification and deciding whether an object was rotated clockwise or counterclockwise). Across different tasks, right parietal TMS impaired orientation judgments, but facilitated object identification, compared to TMS applied to a brain vertex control site. These complementary findings demonstrate that the right parietal lobea region belonging to the dorsal visual streamis critical for processing the spatial attributes of objects, but not their identity. The observed improvement in object recognition, however, suggests an indirect role for the right parietal lobe in object recognition. We propose that this involves the creation of a spatial reference frame for the object, which allows interaction with the object and the individuation of specific viewing instances.

Orientation-invariant object recognition: evidence from repetition blindness

The question of whether object recognition is orientation-invariant or orientation-dependent was investigated using a repetition blindness (RB) paradigm. In RB, the second occurrence of a repeated stimulus is less likely to be reported, compared to the occurrence of a different stimulus, if it occurs within a short time of the first presentation. This failure is usually interpreted as a difficulty in assigning two separate episodic tokens to the same visual type. Thus, RB can provide useful information about which representations are treated as the same by the visual system. Two experiments tested whether RB occurs for repeated objects that were either in identical orientations, or differed by 30, 60, 90, or 180 degrees . Significant RB was found for all orientation differences, consistent with the existence of orientation-invariant object representations. However, under some circumstances, RB was reduced or even eliminated when the repeated object was rotated by 180 degrees , suggesting easier individuation of the repeated objects in this case. A third experiment confirmed that the upside-down orientation is processed more easily than other rotated orientations. The results indicate that, although object identity can be determined independently of orientation, orientation plays an important role in establishing distinct episodic representations of a repeated object, thus enabling one to report them as separate events.

On the failure of distractor inhibition in the attentional blink

We investigated whether a failure of distractor inhibition contributes to the magnitude of theattentional blink (AB). Subjects viewed dual-targetrapid serial visual presentation (RSVP) streams, where the distractors that directly preceded and succeeded Target 2 (T2−1, T2+1) were either identical to each other or different. Previously, Dux, Coltheart, and Harris (2006) found enhanced target report in RSVP due to repetition of distractors around Target 1, which was interpreted as evidence of distractor inhibition. Here, distractor repetition again attenuated the AB, but only at lag 2, the Target 2 position where T2−1 would have undergone attentive processing. Our results demonstrate that the distractor repetition effect is dependent on attention, and that a failure to inhibit distractors contributes to the AB.

Anatomical limitations in mental transformations of body parts

Two experiments investigated whether possible and impossible body postures influence mental rotation processes differently considering that anatomical limitations constrain the way in which subjects perform mental transformations of body parts. In Experiment 1, mental rotation was performed on two stimuli presented simultaneously. Both possible and impossible body postures elicited mental rotation, although the mental rotation rate was slower for impossible postures. In Experiment 2, only one stimulus was presented at a time and subjects decided whether it represented a correct body-part configuration or not. A typical mental rotation function was only present for correct body-part configurations. The results are discussed in terms of the familiarity of the stimuli. Unfamiliar stimuli (physically impossible) are rotated via local representations of their parts, whilst global representations are used for rotating familiar (anatomically correct) stimuli.

On the fate of distractor stimuli in rapid serial visual presentation

Observers demonstrate an impaired ability to report the second of two targets in a rapid serial visual presentation (RSVP) stream if it appears within 500 ms of the first target--a phenomenon known as the attentional blink. This study investigated the fate of stimuli in dual-target RSVP streams that do not require report--the distractors. In five experiments, observers viewed dual-target RSVP streams where the items flanking Target 1 either had the same identity (repeats) or a different identity (non-repeats). Repeated distractors reduced the attentional blink, but only if two conditions were met (1) the items flanking Target 1 were featurally identical and (2) the distractors were drawn from a different alphanumeric category to the targets (e.g. digits vs. letters). We interpret this reduced blink magnitude in the repeat trials as evidence that in RSVP streams distractor items that appear in close temporal proximity to Target 1 are inhibited and we propose that this inhibition occurs both at the level of alphanumeric features and abstract identities.

Disentangling the contributions of grasp and action representations in the recognition of manipulable objects.

There is an increasing evidence that the action properties of manipulable objects can play a role in object recognition, as objects with similar action properties can facilitate each other’s recognition [Helbig et al. Exp Brain Res 174:221–228, 2006]. However, it is unclear whether this modulation is driven by the actions involved in using the object or the grasps afforded by the objects, because these factors have been confounded in previous studies. Here, we attempted to disentangle the relative contributions of the action and grasp properties by using a priming paradigm in which action and grasp similarity between two objects were varied orthogonally. We found that target tools with similar grasp properties to the prime tool were named more accurately than those with dissimilar grasps. However, naming accuracy was not affected by the similarity of action properties between the prime and target tools. This suggests that knowledge about how an object is used is not automatically accessed when identifying a manipulable object. What are automatically accessed are the transformations necessary to interact directly with the object—i.e., the manner in which one grasps the object.