To confirm if DR affected the function of V1 neurons in visual signal processing, we examined the response of V1 neurons to visual stimuli with different orientations and motion directions. 88 neurons in the normal control cats (NC1-4) and 75 neurons in DR cats (DR1-4) were studied in this study (Table 1). All neurons had a receptive field within 8° visual angle from the retinal central area of dominant eye.

The total number of neurons was 75 and 88 respectively for DR cats and control cats. A percentile value indicated the percentage of neurons whose OBs or DBs were lower than the corresponding OB or DB value on the horizontal axis. DR cats showed significantly increased OB and DB value compared with control cats (p<0.0001; p<0.0001).

We first compared the selectivity of V1 neurons for different stimulus orientations and motion directions. The majority of neurons (69.3%) in the control group of cats had an orientation bias (OB) value less than 0.3, whereas 68% neurons in DR group had an OB value larger than 0.3 ( Fig 4A ). As indicated by ANOVA analysis, the mean OB within either control or DR group showed no significant difference between individual cats (Control group: F(3,88) = 0.196, p>0.5; DR group: F(3,75) = 1.681, p>0.1). However, the mean OB of each individual cat in the DR group was significantly larger than that of any individual cat in the control group (Group effect: F(1,163) = 48.206, p<0.0001; Interaction of group and individual cat: F(3,163) = 1.351, p>0.05). Additionally, the average OB value across all cats in the DR group (0.45 ± 0.20) was also significantly higher than in the control group (0.25 ± 0.16) (F(1,163) = 46.725, p<0.0001). Similarly, most of neurons (80.6%) in the control group of cats had a motion direction bias (DB) value smaller than 0.2, whereas more than half (56%) of neurons in the DR group had a DB value higher than 0.2 ( Fig 4B ). The mean DB within the control or DR group exhibited no significant difference between cats (Control group: F(3,88) = 0.363, p>0.5; DR group: F(3,75) = 1.315, p>0.1), whereas the mean DB of each cat in the DR group was significantly larger than that of any individual cat in the control group (Group effect: F(1,163) = 42.705, p<0.0001; Interaction of group and individual cat: F(3,163) = 1.149, p>0.1). The average DB of all cats in the DR group (0.29±0.19) was also significantly higher than that in the control group (0.13±0.10) (F(1,163) = 41.187, p<0.0001). Therefore, we concluded that DR increased the selectivity of V1 neurons for stimulus orientations and motion directions.

Visually-evoked response and spontaneous activity.

The enhanced selectivity of V1 neurons for visual stimulus orientations and motion directions in DR cats could result from an increased response to the optimal stimulus orientation or a decreased response to non-optimal orientations. To clarify this possibility, we compared the neuronal maximum response to its preferred stimulus orientation and the average response to all stimulus orientations between DR group and the control group.

Most of neurons (72.7%) in the control group showed a maximum response (MR) larger than 50 spikes/s, whereas 68% neurons in the DR group displayed a MR smaller than 50 spikes/s (Fig 5A). ANOVA analysis indicated that the mean MR was not significantly different between cats within either DR group or control group (Control group: F(3, 88) = 0.337, p>0.5; DR group: F(3, 75) = 0.189, p>0.5). However, the mean MR value of each individual cat in the DR group was significantly lower when compared with that of any individual cat in the control group (Group effect: F(1, 163) = 33.528, p<0.0001; Interaction of group and cat: F(3, 163) = 0.133, p>0.5). The average MR value across all cats in the DR group (45.6 ± 19.6) was also significantly lower than in the control group (66.4 ± 24.2) (F(1, 163) = 35.557, p<0.0001). Relative to the control group of cats, the average MR in the DR group was lower by 31.3%.

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larger image TIFF original image Download: Fig 5. Percentage of neurons with different range of maximum response (MR) (A), average response (AR) (B), baseline response (BR) (C) and signal-to-noise ratio (STN) (D) for DR and normal control cats. The total number of neurons was 75 and 88 respectively for DR cats and control cats. DR cats showed significantly lower MR, AR and BR (all p<0.0001), but significantly higher STN (p<0.0001) when compared with control cats. https://doi.org/10.1371/journal.pone.0149004.g005

Most of the observed neurons (77.2%) in the control group had an average response (AR) higher than 20 spikes/s, whereas most of neurons (80%) in the DR group had a AR less than 20 spikes/s (Fig 5B). The mean AR displayed no significant difference between cats in either DR or control group (Control group: F(3, 88) = 1.01, p>0.1; DR group: F(3, 75) = 0.715, p>0.5). Nevertheless, the mean AR of each cat in the DR group was significantly lower when compared with that of any individual cat in the control group (Group effect: F(1, 163) = 77.23, p<0.0001; Interaction of group and cat: F(3, 163) = 0.732, p>0.5). The averaged AR of all cats in the DR group (13.6 ± 8.1) was also significantly lower than in the control group (31.3 ± 15.2) (F(1, 163) = 81.592, p<0.0001). Relative to the control group of cats, the average AR in the DR group was lower by 56.5%. Therefore, DR decreased the response of V1 neurons at all stimulus orientations, but the amplitude of the decrease at non-optimal orientations was greater than at the optimal orientation.

To examine whether the spontaneous activity (BR: baseline response) of V1 neurons was also modified by DR, we compared the mean BR value between the two groups of cats. More than half of the neurons (61.3%) in the control group of cats had a BR greater than 6 spikes/s, whereas the majority of the neurons (78.6%) in the DR group had a BR fewer than 4 spikes/s (Fig 5C). ANOVA analysis showed that the average BR of all cats in the DR group (2.8 ± 2.7) was significantly lower than in the control group (9.5 ± 6.7) (F(1, 163) = 65.062, p<0.0001). Further, the mean BR of each cat in the DR group was significantly smaller than that of any individual cat in the control group (Group effect: F(1, 163) = 60.227, p<0.0001; Interaction of group and cat: F(3, 163) = 0.279, p>0.5). Relative to the control group of cats, the average BR in the DR group was 70.3% lower. Because of a large reduction of BR, the signal-to-noise ratio (STN) of V1 neurons in the DR group was significantly higher than that in the control group. The majority of neurons (76.1%) in the control group of cats had a STN value smaller than 20, whereas more than half of neurons (53.3%) in the DR group had a STN higher than 20 (Fig 5D). The averaged STN in the DR group (26.7 ± 20.6) was significantly larger than in the control group (12.8 ± 12.2) (F(1, 163) = 28.446, p<0.0001). Further, the mean STN of each cat in the DR group was significantly higher than of any individual cat in the control group (Group effect: F(1, 163) = 28.932, p<0.0001; Interaction of group and cat: F(3, 163) = 0.377, p>0.5).

All the above analysis demonstrated that DR depressed both spontaneous activity and the visually-evoked response of the V1 neurons, but increased their selectivity and signal-to-noise ratio in response to visual stimuli.