Choongkil Lee

Eye and head coordination in reading: roles of head movement and cognitive control

Simultaneous recording of eye and head movements during reading revealed that head movements consisted of two components: a modulatory-velocity component coupled to eye saccades, and a constant-velocity component that was independent of eye saccades. Whereas the constant-velocity component increased as subjects repeatedly read the same text, neither the magnitude of the modulatory-velocity component, nor the amplitude of the eye movement, increased. This outcome could be closely simulated when the head movement command was assumed to be stronger, and issued earlier with repeated reading. These results suggest that higher-level processes related to text familiarity modulate eye-head coordination through head movements.

Expansion of visual space after saccadic eye movements

Human subjects reported the perceived two-dimensional location of a visual target that was briefly presented after a saccade in the absence of visual references. Consistent with previous studies, immediately after horizontal saccades, there was a salient horizontal component in mislocalization in the direction opposite to the saccade. However, the horizontal component in mislocalization was not constant and was larger for targets presented further into the visual field contraversive to the saccade. For the same horizontal saccades, the vertical component in mislocalization was also obvious, and it was larger for targets located further away from the saccade trajectory. The saccadic effects resulted in an overall pattern of mislocalization that could be best described as a two-dimensional expansion of visual space. The point of expansive origin was not associated with the saccade goal, but was shifted from the saccade goal in the direction of the saccade. These results suggest that spatial information processing at the time of saccades reflects topographic interactions between neural activations from saccade execution and the visual target. The configuration of mislocalized positions of single point stimuli along a line was not comparable to the pattern of non-veridical motion perception described by Park, Lee & Lee (2001), indicating that spatial mislocalization and non-veridical motion perception after saccades are independent phenomena.

Somata-selective lesions induced by cobaltous chloride: A parametric study *

Repeated injections of cobaltous chloride destroy neuronal somata while sparing fibers-of-passage. This effect is dose-dependent and excess cobalt destroys fibers-of-passage as well as somata. By manipulating the concentration and volume of cobaltous chloride injected into rat lateral geniculate nuclei, we defined a range of parameters producing highly selective destruction of somata for regions 1 mm in diameter or smaller.

Head-free reading of horizontally and vertically arranged texts

Parameters of eye and head movements and their coordination in reading horizontally and vertically arranged texts were compared. Reading was faster for horizontally arranged than for vertically arranged texts by 24%, primarily due to larger gaze amplitude for horizontal reading, and thus smaller numbers of saccades and fixations. The higher velocity of gaze saccades for given amplitudes in horizontal than vertical reading also contributed to the difference in reading speed. The horizontal bias in reading is at least partly due to the oculomotor system, because the higher velocity for given amplitude of horizontal saccade was also observed in a control experiment devoid of lexical load, in which a sequentially stepping laser target was tracked. The analysis of instantaneous phase of eye and head movements with a new metric derived by the Hilbert transform suggests that eye and head coupling is stronger for vertical than for horizontal direction in both reading and laser-tracking tasks. These results, combined with previous evidence that text familiarity modulates the timing and strength of head movement commands with respect to eye movements (Vis. Res. 39 (1999) 3761), indicate that the coupling strength between eye and head movements is variable depending on the direction of gaze shift and cognitive context.

Non-veridical visual motion perception immediately after saccades

It is widely assumed that combining the eye movement vector with the motion vector of the retinal image is both sufficient and necessary for recovering the direction and speed of visual motion. Here, we report that execution of a saccadic (rapid) eye movement in the dark systematically biased subsequent perceptual judgment of the direction of visual motion in the direction opposite to the saccade. This non-veridical motion perception reached a maximum immediately after saccade offset and then decayed in approximately equal to 100 ms. These results suggest that the oculomotor signal interacts with central mechanisms related to motion and possibly form perception, as well as spatial vision, as documented with mislocalization of visual objects at the time of saccades.

Properties of saccade-related neurons in the cat superior colliculus: patterns of movement fields and discharge timing

Abstract. The discharge characteristics of saccade-related neurons in the superior colliculi (SC) of trained cats were investigated in order to understand the involvement of the SC in dynamic control of saccades. In particular, two aspects of saccade-related collicular activity in relation to visually guided saccades were quantitatively analyzed: the bounded pattern of amplitude tuning and the timing of neural activity in relation to saccade offset. In order to determine whether the amplitude tuning of SC cells is bounded or unbounded, we characterized it with a quantitative index, the unbounded-field (UF) index. Analysis of the characteristics of the distribution of this index, including the absence of unbounded fields for cells within the central 10°, suggested that all cells form a single class and that apparent unbounded amplitude tuning is an artifact occurring when the range of gaze shift is truncated by restraining the head. This interpretation is supported by the results of a computer simulation based on truncation of gaze-shift range that closely replicated the experimental data. Analyses of relative location of peak motor activity in the spatial and temporal dimensions revealed that the peak discharge was more closely linked to the midpoint of the saccade than to the offset of saccade, regardless of the pattern of amplitude tuning. These results do not support the proposal advanced by the moving-hill hypothesis: the bounded pattern of amplitude tuning is inconsistent with rostral migration of the activity locus during execution of a saccade, and the relative location of the peak motor activity is inconsistent with the idea that the SC activity encodes dynamic motor error (the difference between desired and current gaze directions).

Anatomical characteristics and three‐dimensional model of the dog dorsal lateral geniculate body

The morphological and laminar characteristics of the dorsal lateral geniculate nucleus (LGN) and medial interlaminar nucleus (MIN) of the domestic dog (Canis familiaris) were studied by three‐dimensional computer reconstruction of labeled retinal afferents following intraocular HRP injections. As previously reported, the dog LGN consisted of layers A, A1, C, C1, C2, and C3. Layers A, C, and C2 receive contralateral‐eye inputs, and layers A1 and C1 ipsilateral inputs. The dog MIN was found to have four orderly interdigitating layers; layers 1, 2, 3, and 4, medial to lateral. MIN layers 1 and 3 received contralateral inputs, and layers 2 and 4 ipsilateral inputs. Layer 1 had the largest soma of all LGN/MIN layers. LGN layer A was partially separated into medial and lateral subdivisions by a cleft free of somata. The overall three‐dimensional shape of the lateral geniculate body was like the letter C, with the convex part of the C directed posteriorly. The relative volume of the MIN was smaller than in the cat; the canine MIN comprised 8.3% of the combined volume of layers A, A1 and the MIN, while that of the cat comprised 14.2% as estimated from Sandersons map. The volume of all contralateral‐eye layers, combining both LGN and MIN, was 31.2 mm3 (78%), and that for ipsilateral layers was 8.6 mm3 (22%). The ratio of ipsilateral to contralateral laminar volumes is much lower in the dog than in the cat. Anat Rec 256:29–39, 1999.

Neural discharge coupled to saccade offset in the cat visual cortex

The increase in neural activity in cat visual cortex associated with eye movements has been thought to reflect a replica of the motor command signal. We examined the timing of the saccade-related increase in neural activity in cat areas 17 and 18 in relation to saccade onset and offset. The increase in activity was temporally coupled to saccade offset rather than onset both for visually guided saccades and for spontaneous saccades in the dark. Overall, it occurred 63 ms after saccade offset, and the peak was higher and sharper for data aligned at saccade offset than for onset. These results are inconsistent with the idea that saccade-related activity in cat visual cortex reflects a copy of the motor command signal.

Modulation of V1 spike response by temporal interval of spatiotemporal stimulus sequence.

The spike activity of single neurons of the primary visual cortex (V1) becomes more selective and reliable in response to wide-field natural scenes compared to smaller stimuli confined to the classical receptive field (RF). However, it is largely unknown what aspects of natural scenes increase the selectivity of V1 neurons. One hypothesis is that modulation by surround interaction is highly sensitive to small changes in spatiotemporal aspects of RF surround. Such a fine-tuned modulation would enable single neurons to hold information about spatiotemporal sequences of oriented stimuli, which extends the role of V1 neurons as a simple spatiotemporal filter confined to the RF. In the current study, we examined the hypothesis in the V1 of awake behaving monkeys, by testing whether the spike response of single V1 neurons is modulated by temporal interval of spatiotemporal stimulus sequence encompassing inside and outside the RF. We used two identical Gabor stimuli that were sequentially presented with a variable stimulus onset asynchrony (SOA): the preceding one (S1) outside the RF and the following one (S2) in the RF. This stimulus configuration enabled us to examine the spatiotemporal selectivity of response modulation from a focal surround region. Although S1 alone did not evoke spike responses, visual response to S2 was modulated for SOA in the range of tens of milliseconds. These results suggest that V1 neurons participate in processing spatiotemporal sequences of oriented stimuli extending outside the RF.

Thalamic control of cat area-18 supragranular layers : Simple cells, complex cells, and cells projecting to the lateral suprasylvian visual area

The goal of this study was to determine the effects of inactivating layer A or the C layers of the cat lateral geniculate nucleus on the supragranular layers of area 18, including cells antidromically activated from the lateral suprasylvian visual area (LS). Isolated cells were visually driven via the contralateral eye while the retinotopically corresponding regions of layer A or, in some cases, the C layers were reversibly inactivated with injections of cobaltous chloride. Simple cells were frequently encountered and were on average more dependent upon layer A than were complex cells, a result qualitatively similar to that found previously in area 17 (Malpeli, 1983; Malpeli et al., 1986). However, the influence of the C layers on area 18 was much more apparent than for area 17. In area 18, as in area 17, the dependence of simple cells on particular geniculate layers appears to follow the terminal patterns of the major direct geniculate inputs. Those simple cells most dependent on layer A were located in lower layer 3. Simple cells in upper layer 3, like complex cells, showed little dependence on layer A, but were strongly dependent upon the C layers. All cells antidromically activated from LS were simple cells with rapidly conducting axons. They had, on average, the same moderately strong dependence on layer A as the patches of LS receiving area 18 input (Lee et al., 1997), supporting the conclusion that the influence of layer A in these patches is largely transmitted via association inputs from area 18. These results demonstrate that simple cells play a major role in association pathways.