Thomas N. Pierzchalski appeals the district court’s 1 order affirming the denial of supplemental security income and disability insurance benefits. Upon de novo review of the record, see Davidson v. Astrue, 501 F.3d 987, 989-90 (8th Cir. 2007), we find that the Commissioner’s decision is supported by substantial evidence on the record as a whole. As to Pierzchalski’s *150 arguments for reversal, we find that the administrative law judge (ALJ) gave multiple valid reasons for discounting the opinion of consulting neuropsychologist Vann Smith, including that Dr. Smith saw Pierzchalski only once, see Charles v. Barnhart, 375 F.3d 777, 783 (8th Cir. 2004); we find no evidence of bias on the part of the ALJ, see Rollins v. Massanari, 261 F.3d 853, 857-58 (9th Cir. 2001) (quasi-judicial administrative officers such as ALJs are presumed unbiased, but presumption can be rebutted by showing conflict of interest or other specific reason for disqualification); and we conclude that the ALJ was not required to develop the record further based on Dr. Smith’s new post-hearing diagnosis of cognitive dysfunction, see Gregg v. Barnhart, 354 F.3d 710, 713 (8th Cir. 2003). Accordingly, we affirm.