The Relationship Between Rotary Chair and Video Head Impulse Testing in Children and Young Adults With Cochlear Implants

Purpose Conflicts among video head impulse testing (vHIT) and rotary chair have occurred; therefore, the purpose of this study was to determine the relationship between rotary chair and vHIT outcome parameters to understand when these two tests disagree and determine if one or both test outcomes are needed in children. Method Data from 141 child and young adult subjects (73 males, 68 females, M age = 15 years, range: 6–35) were retrospectively reviewed. Of those, 56 had a cochlear implant and 85 were normal controls. All subjects completed rotary chair and vHIT, which were categorized as (a) normal vestibular function, (b) unilateral vestibular loss, or (c) bilateral vestibular loss. vHIT tracings were analyzed to determine if gain and corrective saccade velocity, frequency, or latency were helpful parameters for determining vestibular loss. Results Of the 141 subjects, the misclassification rate was 13/141 (9%). All normal control subjects were classified as having normal rotary chair and normal vHIT. In subjects with a cochlear implant ( n = 56), the misclassification rate was 13/56 (23%). There were four misclassification patterns. Using rotary chair as the gold standard, receiver operating characteristic analysis revealed optimal cut-points for vHIT gain (< 0.84), corrective saccade frequency (≥ 50%), amplitude (≥ 75°/s), and latency (≤ 320 ms). Using these vHIT cut-points improved the agreement between rotary chair and vHIT, resulting in an overall misclassification rate of 10/141 (7%) and 9/56 (16%) in subjects with a cochlear implant. Conclusions When testing children, caloric testing is often not an option due to tolerability or time. However, discordant results occur between rotary chair and vHIT. These data suggest vHIT is a sufficient first-tier assessment. If abnormal, rotary chair is not necessary. If normal, rotary chair can be helpful for uncovering other indicators of vestibular loss. When interpreting vHIT, including gain and all corrective saccade outcomes may improve sensitivity.

Spelling ability influences early letter encoding during reading: Evidence from return-sweep eye movements

In recent years, there has been an increase in research concerning individual differences in readers’ eye movements. However, this body of work is almost exclusively concerned with the reading of single-line texts. While spelling and reading ability have been reported to influence saccade targeting and fixation times during intra-line reading, where upcoming words are available for parafoveal processing, it is unclear how these variables affect fixations adjacent to return-sweeps. We, therefore, examined the influence of spelling and reading ability on return-sweep and corrective saccade parameters for 120 participants engaged in multiline text reading. Less-skilled readers and spellers tended to launch their return-sweeps closer to the end of the line, prefer a viewing location closer to the start of the next, and made more return-sweep undershoot errors. We additionally report several skill-related differences in readers’ fixation durations across multiline texts. Reading ability influenced all fixations except those resulting from return-sweep error. In contrast, spelling ability influenced only those fixations following accurate return-sweeps—where parafoveal processing was not possible prior to fixation. This stands in contrasts to an established body of work where fixation durations are related to reading but not spelling ability. These results indicate that lexical quality shapes the rate at which readers access meaning from the text by enhancing early letter encoding, and influences saccade targeting even in the absence of parafoveal target information.

Undersweep-fixations during reading in adults and children

Return sweeps take a reader’s fixation from the end of one line to the start of the next. Return sweeps frequently undershoot their target and are followed by a corrective saccade toward the left margin. The pauses prior to corrective saccades are typically considered to be uninvolved in linguistic processing. However, recent findings indicate that these undersweep fixations influence skilled adult readers’ subsequent reading pass across the line and provide a preview of line-initial words. The current research examined these effects in children. First, a children’s reading corpus analysis revealed that words receiving an undersweep fixation were more likely skipped and received shorter gaze durations during a subsequent pass. Second, a novel eye movement experiment that directly compared adults’ and children’s eye movements indicated that, during an undersweep fixation, readers very briefly allocate their attention to the fixated word—as indicated by inhibition of return effects during a subsequent pass—prior to deploying attention toward the line-initial word. We argue that prior to the redeployment of attention, readers extract information at the point of fixation that facilitates later encoding and saccade targeting. Given similar patterns of results for adults and children, we conclude that the mechanisms controlling for oculomotor coordination and attention necessary for reading across line boundaries are established from a very early point in reading development.

Short-latency allocentric control of saccadic eye movements

It is generally accepted that the neural circuits that are implicated in saccade control use retinotopically coded target locations. However, several studies have revealed that nonretinotopic representation is also used. This idea raises a question about whether nonretinotopic coding is egocentric (head or body centered) or allocentric (environment centered). In the current study, we hypothesized that allocentric coding may play a crucial role in immediate saccade control. To test this hypothesis, we used an immediate double-step saccade task toward two sequentially flashed targets with a frame in the background, and we examined whether the end point of the second saccade was affected by a transient shift of the background that participants were told to ignore. When the background was shifted transiently upward (or downward) during the flash of the second target, the second saccade generally erred the target downward (or upward), which was in the direction opposite to the shift of the background. The effect on the second saccade became significant within 150 ms after the frame was presented for decoding and was built up for 200 ms thereafter. When the second saccade was not adjusted, a small, corrective saccade followed within 300 ms. The effect scaled linearly with the shift size up to 3° for a noncorrective second saccade and up to 6° for a corrective saccade. The present results show that an allocentric location of a target is rapidly represented by the brain and used for controlling saccades. NEW & NOTEWORTHY We found that the saccade end point was shifted from the actual target position toward the direction expected from allocentric coding when a large frame in the background was transiently shifted during the period of target presentation. The effect occurred within 150 ms. The present study provides direct evidence that the brain rapidly uses allocentric coding of a target to control immediate saccades.

The Prefrontal Cortex and Oculomotor Delayed Response: A Reconsideration of the “Mnemonic Scotoma”

The concept of the “mnemonic scotoma,” a spatially circumscribed region of working memory impairment produced by unilateral lesions of the PFC, is central to the view that PFC is critical for the short-term retention of information. Presented here, however, are previously unpublished data that offer an alternative, nonmnemonic interpretation of this pattern of deficit. In their study, Wajima and Sawaguchi [Wajima, K., & Sawaguchi, T. The role of GABAergic inhibiton in suppressing perseverative responses in the monkey prefrontal cortex. Neuroscience Research, 50(Suppl. 1), P3–P317, 2004] applied the GABAA antagonist bicuculline methiodide unilaterally to the PFC of two monkeys while they performed an oculomotor delayed-response task. Consistent with previous studies, errors for the initial memory-guided saccade were markedly higher when the cued location fell into the region of the visual field affected by the infusion. These erroneous saccades tended to select an alternative target location (out of a possible 16) that had not been cued on that trial. By extending the analysis window, however, it was observed that the second, “corrective” saccade often acquired the location that had been cued on that trial. Further analysis of the erroneous initial saccades indicated that they tended to be directed to a location that had been relevant on the previous trial. Thus, the deficit was not one of “forgetting” the cued location. Rather, it was one of selecting between currently and previously relevant locations. These findings suggest a need for a reconsideration of the concept of the mnemonic scotoma, which in turn invites a reconsideration of functional interpretations of sustained neuronal activity in PFC.

Control of Predictive Error Correction During a Saccadic Double-Step Task

We explored the nature of control during error correction using a modified saccadic double-step task in which subjects cancelled the initial saccade to the first target and redirected gaze to a second target. Failure to inhibit was associated with a quick corrective saccade, suggesting that errors and corrections may be planned concurrently. However, because saccade programming constitutes a visual and a motor stage of preparation, the extent to which parallel processing occurs in anticipation of the error is not known. To estimate the time course of error correction, a triple-step condition was introduced that displaced the second target during the error. In these trials, corrective saccades directed at the location of the target prior to the third step suggest motor preparation of the corrective saccade in parallel with the error. To estimate the time course of motor preparation of the corrective saccade, further, we used an accumulator model (LATER) to fit the reaction times to the triple-step stimuli; the best-fit data revealed that the onset of correction could occur even before the start of the error. The estimated start of motor correction was also observed to be delayed as target step delay decreased, suggesting a form of interference between concurrent motor programs. Taken together we interpret these results to indicate that predictive error correction may occur concurrently while the oculomotor system is trying to inhibit an unwanted movement and suggest how inhibitory control and error correction may interact to enable goal-directed behaviors.

Complex Spike Activity in the Oculomotor Vermis of the Cerebellum: A Vectorial Error Signal for Saccade Motor Learning?

Brain stem signals that generate saccadic eye movements originate in the superior colliculus. They reach the pontine burst generator for horizontal saccades via short-latency pathways and a longer pathway through the oculomotor vermis (OMV) of the cerebellum. Lesion studies implicate the OMV in the adaptation of saccade amplitude that occurs when saccades become inaccurate because of extraocular muscle weakness or behavioral manipulations. We studied the nature of the possible error signal that might drive adaptation by examining the complex spike (CS) activity of vermis Purkinje (P-) cells in monkeys. We produced a saccade error by displacing the target as a saccade was made toward it; a corrective saccade ∼200 ms later eliminated the resulting error. In most P-cells, the probability of CS firing changed, but only in the error interval between the primary and corrective saccade. For most P-cells, CSs occurred in a tight cluster ∼100 ms after error onset. The probability of CS occurrence depended on both error direction and size. Across our sample, all error directions were represented; most had a horizontal component. In more than one half of our P-cells, the probability of CS occurrence was greatest for error sizes <5° and less for larger errors. In the remaining cells, there was a uniform increased probability of CS occurrence for all errors ≤7–9°. CS responses disappeared when the target was extinguished during a saccade. We discuss the properties of this putative CS error signal in the context of the characteristics of saccade adaptation produced by the target displacement paradigm.

Role of the caudal fastigial nucleus in saccade generation. II. Effects of muscimol inactivation

1. We studied the effect of temporarily inhibiting neurons in the caudal fastigial nucleus in two rhesus macaques trained to make saccades to jumping targets. We placed injections of the gamma-aminobutyric acid (GABA) agonist muscimol unilaterally or bilaterally at sites in the caudal fastigial nucleus where we had recorded saccade-related neurons a few minutes earlier. 2. Unilateral injections (n = 9) made horizontal saccades to the injected side hypermetric and those to the other side hypometric (mean gain of 1.37 and 0.61, respectively, for 10 degrees target steps, and 1.26 and 0.81 for 20 degrees target steps; normal saccade gain was 0.96). Saccades to vertical targets showed a small but significant hypermetria and curved strongly toward the side of the injection. The trajectories and end points of all targeted saccades were more variable than normal. 3. After unilateral injections, centripetal saccades were slightly larger than centrifugal saccades (mean gains for ipsilateral saccades were 1.42 and 1.31, respectively, for 10 degrees target steps, and 1.37 and 1.15 for 20 degrees target steps). 4. Unilateral injections increased the average acceleration of ipsilateral saccades and decreased the acceleration of contralateral saccades. Injections decreased both the acceleration and deceleration of vertical saccades. 5. After dysmetric saccades, monkeys acquired the target with an abnormally high number of hypometric corrective saccades. Injection increased the average number of corrective saccades from 0.6 to 2.1 after 10 degrees horizontal target steps and from 0.8 to 2.1 after 20 degrees steps. The size of each successive corrective saccade in a series decreased, and the latency from the previous corrective saccade increased. 6. Bilateral injections (n = 2) of muscimol, in which we injected first into the left caudal fastigial nucleus and then, within 30 min, into the right, made all saccades hypermetric (mean gain for 10 degrees right, left, up, and down saccades was 1.18, 1.49, 1.43, and 1.10, respectively). Paradoxically, bilateral injection decreased both saccade acceleration and deceleration. Saccade trajectories and end points were more variable than normal. 7. To account for the effects of our injections, we propose that the activity of caudal fastigial neurons on one side normally helps to decelerate ipsilateral saccades and helps to accelerate contralateral saccades by influencing the feedback loop of the saccade burst generator in the brain stem. Without caudal fastigial activity the brain stem burst generator produces hypermetric, variable saccades. We therefore also propose that the influence of caudal fastigial neurons on the burst generator makes saccades more consistent and accurate.(ABSTRACT TRUNCATED AT 400 WORDS)

Short Report: Scrutinization, Spatial Attention, and the Spatial Programming of Saccadic Eye Movements

Results are presented from an experiment in which subjects’ eye movements were recorded while they carried out two visual tasks with similar material. One task was chosen to require close visual scrutiny; the second was less visually demanding. The oculomotor behaviour in the two tasks differed in three ways. (1) When scrutinizing, there was a reduction in the area of visual space over which stimulation influences saccadic eye movements. (2) When moving their eyes to targets requiring scrutiny, subjects were more likely to make a corrective saccade. (3) The duration of fixations on targets requiring scrutiny was increased. The results are discussed in relation to current theories of visual attention and the control of saccadic eye movements.