In the present study, we analyzed data obtained in a previous study conducted by Tamura et al. (2016). In that study, Tamura and colleagues extracellularly recorded the responses of a single neuron in the V1, V4, and IT of four monkeys (Macaca fuscata; two males and two females, body weight 5.9–8.6 kg) to images of natural object surfaces (Fig. 2) to investigate how surface-related features derived from natural objects are represented in the visual cortical areas. All experiments were performed in accordance with the guidelines of the National Institutes of Health (1996) and Japan Neuroscience Society and approved by the Osaka University Animal Experiment Committee.

Stimuli consisting of natural object surfaces used for recording neuronal responses in V1, V4, and IT from M. fuscata by Tamura et al. (2016). A stimulus set in that physiological study consisted of 64 images of eight types of natural objects: stones (St; n = 8, #1–8), tree barks (Ba; n = 8, #9–16), leaves (Le; n = 8, #17–24), flowers (Fl; n = 8, #25–32), fruits and vegetables (FV; n = 8, #33–40), butterfly wings (BW; n = 8, #41–48), feathers (Fe; n = 8, #49–56), and skins and furs (SF; n = 8, #57–64). In that study, the responses of V1, V4, and IT neurons to these images were recorded to investigate how the surface visual features derived from natural objects are presented in these visual cortices. Additionally, these images were provided to the DCNN saliency map model (Pan et al., 2016) to analyze the characteristics of the responses of model neurons.

The experimental procedures were similar to those of their previous study (Tamura et al., 2014). The monkeys were prepared during aseptic surgery, in which a head restraint was implanted. Additionally, the lateral and occipital part of the skull over the recording region was covered with acrylic resin. These surgical procedures were performed under full anesthesia via inhalation of 1–3% isoflurane (Forane, Abbott Japan) in nitrous oxide (70% N2O, 30% O2) through an intratracheal cannula. The monkeys were given an antibiotic (Pentcillin, Toyama Chemical; 40 mg/kg, i.m.), and an anti-inflammatory and analgesic agent (Voltaren, Novartis; or Ketoprofen, Nissin Pharmaceutical) immediately after surgery. The administration of the antibiotic, and anti-inflammatory and analgesic agent was maintained during the first postoperative week. After one to two weeks of recovery, the monkeys’ eyes were examined to enable the selection of appropriate contact lenses that allowed images placed 57 cm from the cornea to be focused on the retina. Photographs of the retinal fundus were used to determine the position of the fovea.

On the day of neural recording, the monkeys were sedated using intramuscular injections of atropine sulfate (0.1 mg/kg) and ketamine hydrochloride (12 mg/kg). During the preparation for neural recording, the monkeys were analgesized via inhalation of 1–3% isoflurane in nitrous oxide (70% N2O, 30% O2) through an intratracheal cannula. These were infused with the opioid fentanyl citrate (Fentanest, Daiichi Sanyo; 0.035 mg/kg/h) in lactated Ringer’s solution. Tamura and colleagues drilled a small hole (∼5 mm) in the resin-covered skull and made a small slit (2 mm) in the dura. They inserted an electrode through the slit to enable the recording of the neuronal responses.

Tamura and colleagues dilated the pupil of the eye contralateral to the recording hemisphere and relaxed the lens of the eye using 0.5% tropicamide/0.5% phenylephrine hydrochloride (Mydrin-P, Santen). They then covered the cornea of the eye with a contact lens of appropriate refractive power and curvature, and an artificial pupil (diameter, 3 mm) so that the eye would focus on images placed 57 cm away. After the electrode for recording the neuronal responses was inserted, they added vecuronium bromide (Masculax, MSD; 0.06 mg/kg/h) to the infusion solution to prevent eye movement during recording. Thus, the monkeys passively viewed stimuli on the display without eye movement.

Tamura and colleagues made single-unit recordings from V1, V4, and IT using a single-shaft electrode with 32 recording probes arranged linearly (A1X32-10 mm 50–413, A1X32-10 mm 100–413; NeuroNexus) or an eight-shaft electrode, where each shaft was a tetrode with four recording probes at the tip arranged in a rhombus (A8X1 tetrode-2 mm 200–312; NeuroNexus), and the centers of adjacent shafts were 0.2 mm apart. The distance between the centers of adjacent recording probes was 50 or 100 μm when using the single-shaft electrode and 25 μm when using the eight-shaft electrode. The activity of a single neuron was isolated offline using custom-made software to avoid the problem caused by spiking activity from the same neuron being recorded by two or more adjacent probes (for details, see Kaneko et al., 1999, 2007; Tamura et al., 2014). The recording sites in V1 were located on the surface of the occipital cortex, well behind the lunate sulcus. Those in V4 were located between the superior temporal sulcus and the lunate sulcus. Those in the IT cortex were located between the superior temporal sulcus and the anterior middle temporal sulcus, and anterior to the posterior middle temporal sulcus. After each recording session, the monkeys received analgesics and antibiotics. Each recording session lasted up to 7 h, and the monkeys had at least a week’s rest between recording sessions.

The stimulus set used by Tamura et al. (2016) consisted of 64 images of eight types of natural objects (Fig. 2): stones (St; n = 8, #1–8), tree barks (Ba; n = 8, #9–16), leaves (Le; n = 8, #17–24), flowers (Fl; n = 8, #25–32), fruits and vegetables (FV; n = 8, #33–40), butterfly wings (BW; n = 8, #41–48), feathers (Fe; n = 8, #49–56), and skins and furs (SF; n = 8, #57–64). The stimuli (6° × 6° in visual angle) were displayed on a liquid crystal display monitor (CG275W, Eizo) that was calibrated via an internal calibrator and checked using a spectrometer (Minolta CS-1000). The luminance values of the white and black areas were 125 and 1.3 cd/m2, respectively. Each stimulus was presented once monocularly for 200 ms against a homogeneous gray background to the eye contralateral to the recording hemisphere, and a homogeneous gray blank screen was presented for intervals of 200 ms between each presentation. This stimulus-presentation procedure was repeated for 25 or 30 blocks during each recording session, with the stimulus order pseudorandomized in each block.

The magnitude of a visually evoked response to a given stimulus was computed based on the firing rate recorded during the 200-ms stimulus-presentation period. To compensate for response latency, the beginning of the 200-ms window of stimulus presentation was shifted to 80 ms after stimulus onset for V1, V4, and IT neurons. The responsiveness of each neuron was qualitatively evaluated by comparing the firing rates recorded during the stimulus-presentation period across stimuli (Kruskal–Wallis test, p <0.01).

The responses of 691 V1 neurons (from two monkeys), 494 V4 neurons (from two monkeys), and 294 IT neurons (from three monkeys) to the 64 images were recorded. In the present study, we compared these responses from V1, V4, and IT neurons with responses in each layer of a DCNN saliency map model. In the experiments conducted by Tamura et al. (2016), the monkeys were analgesized and paralyzed because some of the sessions required >1 h of stable recording. We cannot rule out the possibility that this procedure affected the neuronal responses. However, in previous works, the stimulus selectivity of V1 and IT neurons recorded from anesthetized/paralyzed monkeys was shown to be similar to that of awake-behaving monkeys (Wurtz, 1969; Tamura and Tanaka, 2001), which indicates that any effect of such preparation was likely to be immaterial, if it existed.

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