Optic ataxia and the dorsal visual steam re-visited: Impairment in bimanual haptic matching performed without vision

The 'two visual systems' account proposed by Milner and Goodale (1992) argued that visual perception and the visual control of action depend upon functionally distinct and anatomically separable brain systems: a ventral stream of visual processing that mediates visual perception (object identification and recognition) and a dorsal stream of visual processing mediating visually guided action. Compelling evidence for this proposal was provided by the neuropsychological studies of brain injured patients, in particular the contrasting pattern of impaired and preserved visual processing abilities of the visual object agnostic patient [DF] and optic ataxic patients who it was argued presented with impaired dorsal stream function. Optic ataxia [OA] has thus become a cornerstone of this 'two visual system' account (Pisella et al., 2009). In the current study we re-examine this assumption by investigating how several individuals presenting with OA performed on a bimanual haptic matching task performed without vision, when the bar to be matched was presented haptically or visually. We demonstrate that, unlike neurologically healthy controls who perform the task with high levels of accuracy, all of the optic ataxic patients were unable to perform the task. We interpret this finding as further evidence that the key difficulty experienced by optic ataxic patients across a range of behavioural tasks may be an inability to simultaneously and directly compare two spatial representations so as to compute the difference between them

Supplementary eye field contributions to the execution of saccades to remembered target locations

Multiple lines of evidence indicate that an anatomically discrete region within the dorsal medial frontal cortex - the supplementary eye field (SEF) - is involved in oculomotor control. To delineate this role further, repetitive transcranial magnetic stimulation (rTMS; 10 Hz, 500 ms) was administered either immediately after the presentation of three-step saccade sequences, or, immediately before response execution of memory-guided saccades. In addition, the effects of changes to visuospatial and temporal order demands were examined by contrasting performance in the presence and the absence of target location information. Results revealed that the SEF supports the processing of spatial information relevant to saccade amplitude. Independently of the time of stimulation, saccade gain was reduced by rTMS applied over the SEF, though only when response execution was performed in the absence of target location information. These results provide evidence of a causal role for the SEF in oculomotor control in the absence of visual feedback. © 2008 Elsevier B.V. All rights reserved.</p

Analysis procedure for “Distance to other objects location”.

<p>(A) The vector of the error between reported and original locations can be plotted on two dimensional histograms or heat maps (radius - 10°). This shows symmetric error of memory recall around the original location of the object. (B) A similar analysis was also performed on the vector of distance between the reported location of objects and the original location of all the <u>other objects</u> in the trial. The rightmost heat map shows a similar error distribution, but with reduced frequency, around the original location of other objects. The heat map in the middle shows the chance level for localizing objects around the original location of other objects, computed by taking the trial’s absolute distance of error but randomizing the angular deviation from the original object location.</p

Eye movements in visual search indicate impaired saliency processing in Parkinson's disease

Previous studies have produced contradictory evidence on the nature of the visual search impairment in patients with Parkinson's disease (PD). Eye movements were measured during multi-target search in nine individuals with mild-to-moderate PD. Subjects were asked to click on a response button whenever they judged they were fixating a target for the first time. Compared to age-matched healthy volunteers, PD patients were impaired at efficient search (detecting +s amongst Ls) but not inefficient search (Ts amongst Ls). However, these patients had normal memory for locations as indexed by their rate of re-clicking on previously inspected locations. We suggest that the pattern of gaze for efficient search may reflect impaired saliency processing in PD. © 2008 Elsevier B.V. All rights reserved.</p

Eye movements in visual search indicate impaired saliency processing in Parkinson's disease

Previous studies have produced contradictory evidence on the nature of the visual search impairment in patients with Parkinson's disease (PD). Eye movements were measured during multi-target search in nine individuals with mild-to-moderate PD. Subjects were asked to click on a response button whenever they judged they were fixating a target for the first time. Compared to age-matched healthy volunteers, PD patients were impaired at efficient search (detecting +s amongst Ls) but not inefficient search (Ts amongst Ls). However, these patients had normal memory for locations as indexed by their rate of re-clicking on previously inspected locations. We suggest that the pattern of gaze for efficient search may reflect impaired saliency processing in PD. © 2008 Elsevier B.V. All rights reserved.</p

Supplementary eye field contributions to the execution of saccades to remembered target locations

Multiple lines of evidence indicate that an anatomically discrete region within the dorsal medial frontal cortex - the supplementary eye field (SEF) - is involved in oculomotor control. To delineate this role further, repetitive transcranial magnetic stimulation (rTMS; 10 Hz, 500 ms) was administered either immediately after the presentation of three-step saccade sequences, or, immediately before response execution of memory-guided saccades. In addition, the effects of changes to visuospatial and temporal order demands were examined by contrasting performance in the presence and the absence of target location information. Results revealed that the SEF supports the processing of spatial information relevant to saccade amplitude. Independently of the time of stimulation, saccade gain was reduced by rTMS applied over the SEF, though only when response execution was performed in the absence of target location information. These results provide evidence of a causal role for the SEF in oculomotor control in the absence of visual feedback. © 2008 Elsevier B.V. All rights reserved.</p

Experiment 2: Recall of object identity and object location over two different delays.

<p>(A) Experimental design: similar to experiment 1 but the delay period could be either 1 or 4 seconds, followed by a 2 alternative forced choice between one of the displayed objects and a foil. (B) Object identification performance for the different delays and number of objects in the array for real objects. For 1 object (red) and 4 objects (blue). (C) Localization errors for the different delays, including “nearest object” control (green). (D) Number of swap errors: objects localized within 5° of the original location of other objects (blue) and the number of swap errors as expected from the number of identification failures (grey). Error bars denote SEM across participants.</p

Mean localization error increases with more objects to remember.

<p>(A) Experiment 1: One to 5 <i>o</i>bjects were simultaneously presented at random locations on the screen for 1–5 seconds (1 second per item displayed). Following a delay of 1 second, the objects reappeared in novel random locations and participants were required to “drag” them using the touch screen to their remembered locations. The original locations of objects are shown here in light grey only for illustrative purposes; participants did not receive any feedback as to their errors. (B) Illustration of the dependant variable: distance between the reported locations and their matching original locations. (C) Mean localization error relative to the number of objects presented in the trial. Error bars denote SEM across participants.</p

Number of ‘swapped’ objects increases with memory load.

<p>(A) Percentage of objects localized within 5° of the original location. (B) Percentage of objects localized within 5° of the original location of <i>other objects</i> from the memory array (red). Green line depicts number of objects localized near the original location of other objects expected by chance. (C) Number of objects localized to positions away (5–10°) from original location of other objects. Error bars denote SEM across participants. * p<0.05 and ** p<0.0001, for two tailed paired t tests between the actual and predicted values.</p

Localization error with respect to selection and fixation order.

<p>(A) Error relative to the serial order in which the objects were <u>selected</u> for localization. Different shades represent trials with different numbers of objects. (B) Mean localization error relative to the serial order in which the objects were <u>fixated</u> during the presentation period. The serial order of fixation is calculated according to the first time the object was fixated (left) and the last time it was fixated (right). Note how the error associated with either first or last object to be selected for relocation or fixated during the presentation alters systematically with total number of items in the array. Error bars denote SEM across participants.</p