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Coherent Motion Detection Thresholds and Involuntary Nystagmus
Coherent Motion Detection Thresholds
Author Response: Coherent Motion Detection Thresholds
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Josephine Shallo-Hoffmann, Professor College Of Optometry, Nova Southeastern University
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shoffman{at}nova.edu Josephine Shallo-Hoffmann
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Fu and Boothe1 reported that subsequent to publication of their study2, two of the three alternating monocular occlusion (AMO) subjects (Macaca Mulatta monkeys) had been implanted with dual scleral eye coils for measurement of eye movements under monocular viewing conditions and interpreted the recordings to indicate "that they [the two AMO subjects] have stable eye movements with each eye."1 There seems to be a misunderstanding of what constitutes stability since one of the four eyes showed a low speed horizontal jerk nystagmus. The slow phase had constant velocity waveforms. The average speed of the slow phase was 0.35±0.01 deg/sec (M Mustari, personal communication). Velocity of the motion stimulus in the psychophysical task was 1.81 deg/sec. The involuntary eye movements interfere with the detection of moving stimuli in the psychophysical task and confound the data. Coherent motion detection thresholds were established for all subjects in the study under binocular viewing conditions. The method of constant stimuli was used to test the hypothesis under monocular viewing conditions. The authors dismissed using the method of constant stimuli for determining coherent motion thresholds in the temporal direction for one or both eyes of two monkeys with AMO since the subjects could not reliably perform the task and elected to set coherence threshold for these three eyes at 100%. One of the eyes had nystagmus. Unfortunately, information is lacking concerning the third AMO subject. Until the data from the eyes with nystagmus are removed from the study, the results are ambiguous. Lastly, the oversight of subtle eye movements was detected due to the authors’ careful description of methods and results as well as their (M Mustari, personal communication) forthright presentation of the eye movement data for which the community in this area of study is grateful. Josephine Shallo-Hoffmann College of Optometry
References 1. Fu LN, Boothe RG. Author response: coherent motion detection thresholds. Invest Ophthalmol Vis Sci [eLetter to the Editor]. Available at http://www.iovs.org/cgi/eletters?lookup=by_date&days=9999#33. Accessed January 27, 2003. 2. Fu LN, Boothe RG. A psychophysical measurement and analysis of motion perception in normal and binocularly deprived monkeys. Invest Ophthalmol Vis Sci. 2001;42:2547-2553. |
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Josephine Shallo-Hoffmann, Professor College of Optometry, Nova Southeastern University
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shoffman{at}nova.edu Josephine Shallo-Hoffmann
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Fu and Boothe1 trained monkeys under binocular viewing conditions, three reared under conditions of daily alternating monocular occlusion (AMO) and three raised under conditions of normal visual exposure. The monkeys first learned to distinguish coherent motion (100% correlation) from stationary noise dots and then to distinguish coherent motion (100% correlation) from random dot motion (1.81 deg/sec). At this point in the training, the correlation level of the motion dots was gradually reduced to threshold. On completion of the training, the investigators proceeded to perform the motion detection experiment, under monocular viewing conditions. The investigators reported that one or both eyes of two (of the three) AMO monkeys could not reliably perform the motion detection task at the threshold levels documented during training (i.e., under binocular viewing conditions) and decided to assign threshold at 100% correspondence. Data showed that performance did not exceed 75% correct for any correlation tested for the temporal direction of motion indicating that the data did not provide reliable threshold values. It also suggests that some of the AMO monkeys had nystagmus. Wilson et al., from the same laboratory and with one of the same authors as the study discussed here, used the same subject-species (Mucaca mulatta), and the same daily AMO regimen. They reported that one of their monkeys developed a nystagmus that did not resemble latent nystagmus,2 lending evidence that animals raised under this particular regimen can develop nystagmus. Fu and Boothe argued that the presence of latent nystagmus is not a prerequisite for the development of motion asymmetry. However, they failed to report if the AMO animals in their study had involuntary eye movement, referred to as "asymmetrical eye movement" in their study (Discussion, paragraph 2). The issues concerning MVEPs are not relevant to the fact that any involuntary eye movement will compromise the data. If the AMO monkeys had involuntary eye movement, whether similar or not to any specific nystagmus characteristics,3,4 the eye movement will bias detection of moving stimuli in the psychophysical task and confound the data.5,6 Josephine Shallo-Hoffmann
References 1. Fu LN, Boothe RG. A psychophysical measurement and analysis of motion perception in normal and binocularly deprived monkeys. Invest Ophthalmol Vis Sci. 2001;42:2547-2553. 2. Wilson JR, Noyd WW, Aiyer AD, Norcia AM, Mustari MJ, Boothe RG. Asymmetric responses in cortical visually evoked potentials to motion are not derived from eye movements. Invest Ophthalmol Vis Sci. 1999;40:2435-2499. 3. Tusa RJ, Repka MX, Smith CB, Herdman SJ. Early visual deprivation results in persistent strabismus and nystagmus in monkeys. Invest Ophthalmol Vis Sci. 1991;32:134-141. 4. Tusa RJ, Mustari MJ, Burrows AF, Fuchs AF. Gaze-stabilizing deficits and latent nystagmus in monkeys with brief, early-onset visual deprivation: eye movement recordings. J Neurophysiol. 2001;86:651-661. 5. Shallo-Hoffmann J, Faldon M, Hague S, Riordan-Eva P, Fells P, Gresty M. Motion detection deficits in infantile esotropia without nystagmus. Invest Ophthalmol Vis Sci. 1997;38:219-226. 6. Shallo-Hoffmann JA, Bronstein AM, Acheson J, Morland AB, Gresty MA. Vertical and horizontal motion perception in congenital nystagmus. Neuro-ophthalmology. 1998;19:171-183. |
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Ronald G. Boothe, Professor Emory University
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boothe{at}rmy.emory.edu Ronald G. Boothe
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Shallo-Hoffmann raises an important issue about whether the psychophysical results reported in our paper1 might have been influenced by abnormal eye movements. We presented several lines of indirect evidence in our paper that argue against this possibility. However, at the time we published our paper we did not have direct evidence regarding gaze stability in our subjects. Subsequently, two of the AMO monkeys reported in our study have been implanted with dual scleral eye coils for measurement of eye movements and have head stabilization hardware for prevention of head movements. We recorded gaze holding under monocular viewing conditions and found that they have stable eye movements with each eye. We also did the cover and uncover test on the same monkeys. These tests revealed strabismus but no latent nystagmus. Shallo-Hoffmann refers to results of several other monkeys published previously, including some by our research group.2-5 The monkeys in these studies that developed latent nystagmus3,4 had different rearing conditions than our AMO monkeys. These monkeys were binocularly deprived for either the first 25 or 55 days of life. Monkeys reared under AMO conditions develop strabismus, and occasionally unidirectional nystagmus, but not latent nystagmus.2,5 The psychophysical deficits we reported were only for temporally directed motion, while performance during nasalward motion was unaffected. Lai Ngor Fu
Emory University References 1. Fu LN, Boothe RG. A psychophysical measurement and analysis of motion perception in normal and binocularly deprived monkeys. Invest Ophthalmol Vis Sci. 2001;42:2547-2553. 2. Wilson JR, Noyd WW, Aiyer AD, Norcia AM, Mustari MJ, Boothe RG. Asymmetric responses in cortical visually evoked potentials to motion are not derived from eye movements. Invest Ophthalmol Vis Sci. 1999;40:2435-2439. 3. Tusa RJ, Repka MX, Smith CB, Herdman SJ. Early visual deprivation results in persistent strabismus and nystagmus in monkeys. Invest Ophthalmol Vis Sci. 1991;32:134-141. 4. Tusa RJ, Mustari MJ, Burrows AF, Fuchs AF. Gaze-stabilizing deficits and latent nystagmus in monkeys with brief, early-onset visual deprivation: eye movement recordings. J Neurophysiol. 2001;86:651-661. 5. Burrows AF, Economidies JR, Mustari MJ, Tusa RJ, Boothe RG. Simultaneous binocular experience is not necessary to develop gaze holding or binocualar sensitivity in the pretectal nucleus of the optic tract (NOT) and extrastriate cortex. Society for Neuroscience Abstracts, 660.1, Vol 25, 1999. |
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