Interaction between affordance and handedness recognition: a chronometric study

The visualization of tools and manipulable objects activates motor-related areas in the cortex, facilitating possible actions toward them. This pattern of activity may underlie the phenomenon of object affordance. Some cortical motor neurons are also covertly activated during the recognition of body parts such as hands. One hypothesis is that different subpopulations of motor neurons in the frontal cortex are activated in each motor program; for example, canonical neurons in the premotor cortex are responsible for the affordance of visual objects, while mirror neurons support motor imagery triggered during handedness recognition. However, the question remains whether these subpopulations work independently. This hypothesis can be tested with a manual reaction time (MRT) task with a priming paradigm to evaluate whether the view of a manipulable object interferes with the motor imagery of the subject's hand. The MRT provides a measure of the course of information processing in the brain and allows indirect evaluation of cognitive processes. Our results suggest that canonical and mirror neurons work together to create a motor plan involving hand movements to facilitate successful object manipulation.

Interaction between affordance and handedness recognition: a chronometric study.

The visualization of tools and manipulable objects activates motor-related areas in the cortex, facilitating possible actions toward them. This pattern of activity may underlie the phenomenon of object affordance. Some cortical motor neurons are also covertly activated during the recognition of body parts such as hands. One hypothesis is that different subpopulations of motor neurons in the frontal cortex are activated in each motor program; for example, canonical neurons in the premotor cortex are responsible for the affordance of visual objects, while mirror neurons support motor imagery triggered during handedness recognition. However, the question remains whether these subpopulations work independently. This hypothesis can be tested with a manual reaction time (MRT) task with a priming paradigm to evaluate whether the view of a manipulable object interferes with the motor imagery of the subject's hand. The MRT provides a measure of the course of information processing in the brain and allows indirect evaluation of cognitive processes. Our results suggest that canonical and mirror neurons work together to create a motor plan involving hand movements to facilitate successful object manipulation.

Postura da mão e imagética motora: um estudo sobre reconhecimento de partes do corpo / Hand posture and motor imagery: a body-part recognition study

OBJETIVOS: Assim como a imagética motora, o reconhecimento de partes do corpo aciona representações somatosensoriais específicas. Essas representações são ativadas implicitamente para comparar o corpo com o estímulo. No presente estudo, investigou-se a influência da informação proprioceptiva da postura no reconhecimento de partes do corpo (mãos) e propõe-se a utilização dessa tarefa na reabilitação de pacientes neurológicos. MATERIAIS E MÉTODOS: Dez voluntários destros participaram do experimento. A tarefa era reconhecer a lateralidade de figuras da mão apresentada, em várias perspectivas e em vários ângulos de orientação. Para a figura da mão direita, o voluntário pressionava a tecla direita e para a figura da mão esquerda, a tecla esquerda. Os voluntários realizavam duas sessões: uma com as mãos na postura prona e outra com as mãos na postura supina. RESULTADOS: Os tempos de reação manual (TRM) eram maiores para as vistas e orientações, nas quais é difícil realizar o movimento real, mostrando que durante a tarefa, existe um acionamento de representações motoras para comparar o corpo com o estímulo. Além disso, existe uma influência da postura do sujeito em vistas e ângulos específicos. CONCLUSÕES: Estes resultados mostram que representações motoras são ativadas para comparar o corpo com o estímulo e que a postura da mão influencia esta ressonância entre estímulo e parte do corpo.

OBJECTIVE: Recognition of body parts activates specific somatosensory representations in a way that is similar to motor imagery. These representations are implicitly activated to compare the body with the stimulus. In the present study, we investigate the influence of proprioceptive information relating to body posture on the recognition of body parts (hands). It proposes that this task could be used for rehabilitation of neurological patients. METHODS: Ten right-handed volunteers participated in this experiment. The task was to recognize the handedness of drawings of a hand that were presented in different perspectives and several orientations. For drawings of a right hand, the volunteers pressed the right key, and for drawings of a left hand, they pressed the left key. The volunteers underwent two sessions: one with their hands in a prone posture and the other with their hands in a supine posture. RESULTS: The manual reaction time was longer for perspectives and orientations for which the real movement was difficult to achieve. This showed that, during the task, motor representations were activated to compare the body with the stimulus. Furthermore, the subject's posture had an influence in relation to specific perspectives and orientations. CONCLUSIONS: These results showed that motor representations are activated to compare the body with the stimulus, and that the position of the hand influences this resonance between the stimulus and the body part.

Mental rotation of anthropoid hands: a chronometric study.

It has been shown that mental rotation of objects and human body parts is processed differently in the human brain. But what about body parts belonging to other primates? Does our brain process this information like any other object or does it instead maximize the structural similarities with our homologous body parts? We tried to answer this question by measuring the manual reaction time (MRT) of human participants discriminating the handedness of drawings representing the hands of four anthropoid primates (orangutan, chimpanzee, gorilla, and human). Twenty-four right-handed volunteers (13 males and 11 females) were instructed to judge the handedness of a hand drawing in palm view by pressing a left/right key. The orientation of hand drawings varied from 0 masculine (fingers upwards) to 90 masculine lateral (fingers pointing away from the midline), 180 masculine (fingers downwards) and 90 masculine medial (finger towards the midline). The results showed an effect of rotation angle (F(3, 69) = 19.57, P < 0.001), but not of hand identity, on MRTs. Moreover, for all hand drawings, a medial rotation elicited shorter MRTs than a lateral rotation (960 and 1169 ms, respectively, P < 0.05). This result has been previously observed for drawings of the human hand and related to biomechanical constraints of movement performance. Our findings indicate that anthropoid hands are essentially equivalent stimuli for handedness recognition. Since the task involves mentally simulating the posture and rotation of the hands, we wondered if "mirror neurons" could be involved in establishing the motor equivalence between the stimuli and the participants' own hands.

Mental rotation of anthropoid hands: a chronometric study

It has been shown that mental rotation of objects and human body parts is processed differently in the human brain. But what about body parts belonging to other primates? Does our brain process this information like any other object or does it instead maximize the structural similarities with our homologous body parts? We tried to answer this question by measuring the manual reaction time (MRT) of human participants discriminating the handedness of drawings representing the hands of four anthropoid primates (orangutan, chimpanzee, gorilla, and human). Twenty-four right-handed volunteers (13 males and 11 females) were instructed to judge the handedness of a hand drawing in palm view by pressing a left/right key. The orientation of hand drawings varied from 0° (fingers upwards) to 90° lateral (fingers pointing away from the midline), 180° (fingers downwards) and 90° medial (finger towards the midline). The results showed an effect of rotation angle (F(3, 69) = 19.57, P < 0.001), but not of hand identity, on MRTs. Moreover, for all hand drawings, a medial rotation elicited shorter MRTs than a lateral rotation (960 and 1169 ms, respectively, P < 0.05). This result has been previously observed for drawings of the human hand and related to biomechanical constraints of movement performance. Our findings indicate that anthropoid hands are essentially equivalent stimuli for handedness recognition. Since the task involves mentally simulating the posture and rotation of the hands, we wondered if "mirror neurons" could be involved in establishing the motor equivalence between the stimuli and the participants' own hands.

Experimental context modulates warning signal effects.

Previous studies have shown that saccadic eye responses but not manual responses were sensitive to the kind of warning signal used, with visual onsets producing longer saccadic latencies compared to visual offsets. The aim of the present study was to determine the effects of distinct warning signals on manual latencies and to test the premise that the onset interference, in fact, does not occur for manual responses. A second objective was to determine if the magnitude of the warning effects could be modulated by contextual procedures. Three experimental conditions based on the kind of warning signal used (visual onset, visual offset and auditory warning) were run in two different contexts (blocked and non-blocked). Eighteen participants were asked to respond to the imperative stimulus that would occur some milliseconds (0, 250, 500 or 750 ms) after the warning signal. The experiment consisted in three experimental sessions of 240 trials, where all the variables were counterbalanced. The data showed that visual onsets produced longer manual latencies than visual offsets in the non-blocked context (275 vs 261 ms; P < 0.001). This interference was obtained, however, only for short intervals between the warning and the stimulus, and was abolished when the blocked context was used (256 vs 255 ms; P = 0.789). These results are discussed in terms of bottom-up and top-down interactions, mainly those related to the role of attentional processing in cancelling out competitive interactions and suppressive influences of a distractor on the relevant stimulus.

Experimental context modulates warning signal effects

Previous studies have shown that saccadic eye responses but not manual responses were sensitive to the kind of warning signal used, with visual onsets producing longer saccadic latencies compared to visual offsets. The aim of the present study was to determine the effects of distinct warning signals on manual latencies and to test the premise that the onset interference, in fact, does not occur for manual responses. A second objective was to determine if the magnitude of the warning effects could be modulated by contextual procedures. Three experimental conditions based on the kind of warning signal used (visual onset, visual offset and auditory warning) were run in two different contexts (blocked and non-blocked). Eighteen participants were asked to respond to the imperative stimulus that would occur some milliseconds (0, 250, 500 or 750 ms) after the warning signal. The experiment consisted in three experimental sessions of 240 trials, where all the variables were counterbalanced. The data showed that visual onsets produced longer manual latencies than visual offsets in the non-blocked context (275 vs 261 ms; P < 0.001). This interference was obtained, however, only for short intervals between the warning and the stimulus, and was abolished when the blocked context was used (256 vs 255 ms; P = 0.789). These results are discussed in terms of bottom-up and top-down interactions, mainly those related to the role of attentional processing in canceling out competitive interactions and suppressive influences of a distractor on the relevant stimulus.

Inhibition of return, gap effect and saccadic reaction time to a visual target.

Simple manual reaction time (MRT) to a visual target (S2) is shortened when a non-informative cue (S1) is flashed at the S2 location shortly before the onset of S2 (early facilitation). Afterwards, MRT to S2 appearing at the S1 location is lengthened (inhibition of return - IOR). Similar results have been obtained for saccadic reaction time (SRT). Moreover, when there is a temporal gap between offset of the fixation point (FP) and onset of a target (gap paradigm), SRT is shorter than SRT in an overlap paradigm (FP remains on). In the present study, we determined SRT to S2 (10 degrees) after presenting S1 at the same eccentricity (10 degrees) or at a parafoveal position (2 degrees) in the same or in the opposite hemifield. In addition, we employed both gap and overlap paradigms. Twelve subjects were asked not to respond to S1 (2 degrees or 10 degrees) to the right or to the left of FP, but to respond by making a saccadic movement in response to S2. We obtained the following results: 1) a 40-ms gap effect, 2) an interaction between gap effect and IOR, 3) a 39-ms delay (IOR) when S2 appeared at the cued (S1) position, and 4) a smaller (17 ms) but significant inhibition when S1 occurred at 2 degrees in the ipsilateral hemifield. Thus, a parafoveal (2 degrees) S1 elicits an inhibition of SRT towards ipsilateral peripheral targets. Since an inhibition of the ipsilateral hemifield by a 1 degree eccentric cue has been reported to occur when manual responses are employed, we suggest that the postulated functional link between covert and overt orienting of attention is also valid for parafoveal cues.

Inhibition of return, gap effect and saccadic reaction time to a visual target

Simple manual reaction time (MRT) to a visual target (S2) is shortened when a non-informative cue (S1) is flashed at the S2 location shortly before the onset of S2 (early facilitation). Afterwards, MRT to S2 appearing at the S1 location is lengthened (inhibition of return - IOR). Similar results have been obtained for saccadic reaction time (SRT). Moreover, when there is a temporal gap between offset of the fixation point (FP) and onset of a target (gap paradigm), SRT is shorter than SRT in an overlap paradigm (FP remains on). In the present study, we determined SRT to S2 (10º) after presenting S1 at the same eccentricity (10º) or at a parafoveal position (2º) in the same or in the opposite hemifield. In addition, we employed both gap and overlap paradigms. Twelve subjects were asked not to respond to S1 (2º or 10º) to the right or to the left of FP, but to respond by making a saccadic movement in response to S2. We obtained the following results: 1) a 40-ms gap effect, 2) an interaction between gap effect and IOR, 3) a 39-ms delay (IOR) when S2 appeared at the cued (S1) position, and 4) a smaller (17 ms) but significant inhibition when S1 occurred at 2º in the ipsilateral hemifield. Thus, a parafoveal (2º) S1 elicits an inhibition of SRT towards ipsilateral peripheral targets. Since an inhibition of the ipsilateral hemifield by a 1º eccentric cue has been reported to occur when manual responses are employed, we suggest that the postulated functional link between covert and overt orienting of attention is also valid for parafoveal cues.

Gap effect and reaction time distribution: simple vs choice manual responses

It is well known that saccadic reaction times (SRT) are reduced when the target is preceded by the offset of the fixation point (FP) - the gap effect. Some authors have proposed that the FP offset also allows the saccadic system to generate a separate population of SRT, the express saccades. Nevertheless, there is no agreement as to whether the gap effect and express responses are also present for manual reaction times (MRT). We tested the gap effect and the MRT distribution in two different conditions, i.e., simple and choice MRT. In the choice MRT condition, subjects need to identify the side of the stimulus and to select the appropriate response, while in the simple MRT these stages are not necessary. We report that the gap effect was present in both conditions (22 ms for choice MRT condition; 15 ms for simple MRT condition), but, when analyzing the MRT distributions, we did not find any clear evidence for express manual responses. The main difference in MRT distribution between simple and choice conditions was a shift towards shorter values for simple MRT.

Gap effect and reaction time distribution: simple vs choice manual responses.

It is well known that saccadic reaction times (SRT) are reduced when the target is preceded by the offset of the fixation point (FP)--the gap effect. Some authors have proposed that the FP offset also allows the saccadic system to generate a separate population of SRT, the express saccades. Nevertheless, there is no agreement as to whether the gap effect and express responses are also present for manual reaction times (MRT). We tested the gap effect and the MRT distribution in two different conditions, i.e., simple and choice MRT. In the choice MRT condition, subjects need to identify the side of the stimulus and to select the appropriate response, while in the simple MRT these stages are not necessary. We report that the gap effect was present in both conditions (22 ms for choice MRT condition; 15 ms for simple MRT condition), but, when analyzing the MRT distributions, we did not find any clear evidence for express manual responses. The main difference in MRT distribution between simple and choice conditions was a shift towards shorter values for simple MRT.

Interaction between facilitatory and inhibitory effects due to voluntary and automatic covert orienting of attention.

Covert orienting of attention to one spatial location improves the processing of signals occurring at this location at the expenses of the processing of signals occurring at other spatial positions. According to the premotor theory of visual attention, the voluntary orienting of attention to a peripheral position corresponds to the programming of a saccadic eye movement towards this position. A similar mechanism has been proposed to explain the inhibitory effects elicited by a non-informative peripheral cue. This review discusses some neural mechanisms involved in the facilitatory and inhibitory effects due to covert orienting of attention.

Interaction between facilitatory and inhibitory effects due to voluntary and automatic covert orienting of attention

Covert orienting of attention to one spatial location improves the processing of signals occurring at this location at the expenses of the processing of signals occurring at other spatial positions. According to the premotor theory of visual attention, the voluntary orienting of attention to a peripheral position corresponds to the programming of a saccadic eye movement towards this position. A similar mechanism has been proposed to explain the inhibitory effects elicited by a non-informative peripheral cue. This review discusses some neural mechanisms involved in the facilitatory and inhibitory effects due to covert orienting of attention.

ONSET and OFFSET inhibitions: effect of increase and decrease of cue luminance on manual reaction time to a visual target.

Simple reaction time (RT) to a peripheral visual target is shortened when a non-informative cue is flashed at the target location 100-150 ms before target onset (early facilitation). With longer intervals, RT to targets appearing at cue hemifield is lengthened (inhibition of return). In the present study, we investigated these effects inverting the stimulus contrast in relation to background to see how these effects are related to the onset and/or to the offset of a cue darker or brighter than background. Ten subjects were asked not to respond to a non-informative cue (S1) appearing on a computer screen 6 degrees to the right or to the left of the center of a fixation cross (FP), but to respond, by pressing a microswitch, to a target (S2) occurring at 4 degrees from the FP in the same hemifield as S1 or in the opposite hemifield. There were two different types of sessions. In one, S1 and S2 were bright against a dark background and in the other, S1 and S2 were dark against a bright background. In each session there were two types of trials. In OFF trials, each trial began with the presentation of FP. Five hundred ms later, S1 appeared and remained on for 700 ms. S2 appeared 100 or 800 ms after the offset of S1. In ON trials, S1 onset occurred 1200 ms after the beginning of the trial and remained on until the end of trial. S2 appeared 100 or 800 ms after S1 onset.(ABSTRACT TRUNCATED AT 250 WORDS)

Onset and offset inhibitions: effect of increase and decrease of cue luminance on manual reaction time to a visual target

Simple reaction time (RT) to a peripheral visual target is shortened when a non-informative cue is flashed at the target location 100-150 ms before target onset (early facilitation). With longer intervals, RT to targets appearing at cue hemifield is lengthened (inhibition of return). In the present study, we investigated these effects inverting the stimulus contrast in relation to background to see how these effects are related to the onset and/or to the offset of a cue darker or brighter than background. Ten subjects were asked not to respond to a non-informative cue (S1) appearing on a computer screen 6§ to the right or to the left of the center of a fixation cross (FP), but to respond, by pressing a microswitch, to a target (S2) occurring at 4§ from the FP in the same hemifield as S1 or in the opposite hemifield. There were two different types of sessions. In one, S1 and S2 were bright against a dark background and in the other, S1 and S2 were dark against a bright background. In each session there were two types of trials. In OFF trials, each trial began with the presentation of FP. Five hundred ms later, S1 appeared and remained on for 700 ms. S2 appeared 100 or 800 ms after the offset of S1. In ON trials, S1 onset occurred 1200 ms after the beginning of the trial and remained on until the end of trial. S2 appeared 100 or 800 ms after S1 onset. When S2 onset followed S1 onset by 100 ms, RT to S2 occuring in the same hemifield did not differ from RT when S1 and S2 were in opposite hemifields. In contrast, after an S1-S2 interval of 800 ms, S1 onset elicited an inhibition of its hemifield. This inhibition was similar to that observed 100 ms or 800 ms after S1 offset. The same results were obtained if the cue and target were brighter or darker than the background, showing that the ONSET and OFFSET inhibitions are related not to cue luminance increase and decrease, respectively, but to the appearance and the disappearance of a salient stimulus in a homogeneous background. Moreover, these results suggest that ON and OFF channels of the visual system have similar effects on the orienting of attention

Onset and offset of a visual cue have different effects on manual reaction time to a visual target.

Simple reaction time (RT) to a peripheral visual target is shortened when a non-informative cue is flashed at target location 100-150 ms before target onset (early facilitation). Afterwards, RT to targets appearing at cue location is lengthened (inhibition of return). In the present study we have investigated if these effects arise from the onset and/or from the offset of the cue and the time-dependence of these effects. Twelve subjects were asked not to respond to a non-informative cue (S1) appearing on a computer screen 6 degrees to the right or to the left of a fixation point (FP), but to respond, by pressing a key, to a target (S2) occurring at 4 degrees from the FP in the same hemifield as S1 or in the opposite hemifield. There were two different types of trials. In both, a brief auditory stimulus (W) occurring 700 ms after the onset of FP warned the subject that S2 would appear 100, 200, 300, 500 or 800 ms later. Trials where the onset of S1 coincides with W and S1 remains on until the response to S2 are called ON trials. In OFF trials, S1 onset occurs at the beginning of the trial and its offset coincides with W. We found that in ON trials, RTs to S2 occurring ipsi- or contralaterally to S1 did not differ. In contrast, S1 offset elicited an inhibition of its hemifield beginning at least 100 ms after S1 offset and extending up to 800 ms.

Spatial distribution of the inhibition elicited by the offset of a visual cue on manual reaction time to a visual target.

Simple reaction time (RT) to a peripheral visual target (S2) is shortened when a non-informative cue (S1) is flashed at the S2 location 100-150 ms before target onset (early facilitation). Afterwards, RTs to targets appearing at the S1 location are lengthened (inhibition of return). In the present investigation we studied the spatial distribution of the inhibition elicited by the offset of S1. Twelve subjects were asked not to respond to S1 which appeared on a horizontal meridian located 5.5 degrees above the fixation point (FP), but to respond, by pressing a key, to a target (S2) occurring at 5.5 degrees to the left or to the right. S1 could appear at one of 9 locations along this meridian (5.5, 3.5, 1.5, and 0.5 degrees to the left, 0.0 and 0.5, 1.5, 3.5, and 5.5 degrees to the right) and S2 occurred only at the most eccentric positions. Each trial began with the presentation of FP. Five-hundred ms later, S1 appeared and remained on for 700 ms. One hundred or 800 ms after S1 offset, S2 appeared in the same or in the opposite hemifield. We found that the offset of S1 elicits an inhibition (OFF-inhibition) which has the following features: a) it is maximal at cue's position; b) it spreads to other positions in the cued hemifield, and c) it decreases when the time interval between S1 offset and S2 onset increases from 100 to 800 ms.

Onset and offset of a visual cue have different effects on manual reaction time to a visual target

Simple reaction time (RT) to a peripheral visual target is shortened when a non-informative cue is flashed at target location 100-150 ms before target onset (early facilitation). Afterwards, RT to targets appearing at cue location is lengthened (inhibition of return). In the present study we have investigated if these effects arise from the onset and/or from the offset of the cue and the time-dependence of these effects. Twelve subjects were asked not to respond a non-informative cue (S1) appearing on a computer screen 6§ to the right or to the left of a fixation point (FP), but to respond, by pressing a key, to a target (S2) occurring at 4§ from the FP in the same hemifield as S1 or in the opposite hemifield. There were two different types of trials. In both, a brief auditory stimulus (W) occurring 700 ms after the onset of FP warned the subject that S2 would appear 100, 200, 300, 500 or 800 ms later. Trials where the onset of S1 coincides with W and S1 remains on until the response to S2 are called ON trials. In OFF trials, S1 onset occurs at the beginning of the trial and its offset coincides with W. We found that in On trials, RTs to S2 occurring ipsi-or contralaterally to S1 did not differ. In contrast, S1 offset elicited an inhibition of its hemifield beginning at least 100 ms after S1 offset and extending up to 800 ms

Spatial distribution of the inhibition elicited by the offset of a visual cue on manual reaction time to a visual target

Simple reaction time (RT) to a peripheral visual target (S2) is shortened when a non-informative cue (S1) is flashed at the S2 location 100-150 ms before target onset (early facilitation). Afterwards, RTs to targets appearing at the S1 location are lengthened (inhbition of return). In the present investigation we studied the spatial distribution of the inhibition elicited by the offset of S1. Twelve subjects were asked not to respond to S1 which appeared on a horizontal meridian located 5.5§ above the fixation point (FP), but to respond, by pressing a key, to a target (S2) occurring at 5.5§ to the left or to the right. S1 could appear at one of 9 locations along this meridian (5.5, 3.5, 1.5, and 0.5§ to the left, 0.0 and 0.5, 1.5, 3.5, and 5.5§ to the right) and S2 occurred only at the most eccentric positions. Each trial began with the presentation of FP. Five-hundred ms later, S1 appeared and remained on for 700 ms. One hundred or 800 ms after S1 offset, S2 appeared in the same or in the opposite hemifield. We found that the offset of S1 elicits an inhibition (OFF-inhibition) which has the following features: a) it is maximal at cue's position; b) it spreads to other positions in the cued hemifield, and c) it decreases when the time interval between S1 offset and S2 onset increases from 100 to 800 ms

The retinal distribution of ganglion cells with crossed and uncrossed projections and the visual field representation in the opossum.

The retinal distribution of ganglion cells with crossed and uncrossed projections in the South American opossum, Didelphis marsupialis, was revealed by delivering HRP to one optic tract or to retinal targets of one hemisphere. The cells with uncrossed projections are restricted to the temporal retina, comprising 1/3 of the total retinal area, with a sharp transition at the naso-temporal boundary. Besides being distributed over the nasal 2/3 of the retina, cells with crossed projections are intermingled with those with uncrossed projections over the entire temporal retina. Quantitative analysis about the representation of the horizontal meridian on four specimens revealed that the maximum density of cells with uncrossed projections is on the average located at 3.2 mm (SD = 0.21), i.e. 34.8 deg, temporal to the optic disk, falling to 10% at 2.1 mm (SD = 0.14) or 22.8 deg. On the other hand, the peak for cells with crossed projections is more nasally placed at 1.8 mm (SD = 0.18), i.e. 19.6 deg. Between these two maxima, the site wherein the densities of cells with crossed and uncrossed projections are about equal is on the average about 2.7 mm (SD = 0.25) form the optic disk, i.e. 29.3 deg. This estimate supports the hypothesis that the retinal intersection of the vertical meridian lies within the region of split representation of crossed and uncrossed ganglion cells. In addition, it was observed that the opossum's retina has a large contingent of cells with uncrossed projections temporal to an eccentricity of 2.7 mm from the optic disk, where it represents roughly 2/3 of the ganglion cells. These data corroborate the relevance of the opossum as a non-primate model for visual work.